text:
Is a belief in rebirth a necessity for Buddhist practice?
Can someone be a wholehearted Buddhist practitioner if they entirely reject rebirth? Or is anyone that follows the Buddhist path while not subscribing to rebirth only getting half the story? Or is the non-rebirth believer deluding themselves if they follow the Dharma under that circumstance?
Is the principle of rebirth lacking emphasis in some traditions perhaps? Or is it perhaps something that was culturally prevalent at the time of the Buddha but now can be safely disregarded? Or is it central to the enlightenment experience? Certainly the Buddha reports seeing clearly all his past lives on the occasion of his enlightenment.
So in short is a belief in rebirth a necessity for Buddhist practice?

Unless you have proven to yourself either way, why believe either way? Either way you could be wrong!

No tradition, except maybe Secular Buddhism, rejects literal rebirth, it is a core part of the dhamma. I will say though that there is evidence the Buddha was never " believe in rebirth or else."
Probably the most famous Sutta for Western Buddhism is also the most over rated, misquoted and misunderstood. That being the Kalama Sutta. It is most famous for its paragraph talking about not going by reports or because someone says something.
What is much more profound, and more important imo, is the lesser known section towards the bottom of the sutta, where the Buddha says... ok what IF there is no rebirth and kamma..
http://www.accesstoinsight.org/tipitaka/an/an03/an03.065.than.html
Kalama Sutta: To the Kalamas

"So it is, Blessed One. So it is, O One Well-gone. One who is a
  disciple of the noble ones — his mind thus free from hostility, free
  from ill will, undefiled, & pure — acquires four assurances in the
  here-&-now:
"'If there is a world after death, if there is the fruit of actions
  rightly & wrongly done, then this is the basis by which, with the
  break-up of the body, after death, I will reappear in a good
  destination, the heavenly world.' This is the first assurance he
  acquires.
"'But if there is no world after death, if there is no fruit of actions rightly & wrongly done, then here in the present life I look
  after myself with ease — free from hostility, free from ill will, free
  from trouble.' This is the second assurance he acquires."

Now does the Buddha in multiple places basically say that believing there is no next world, no fruit, etc, is wrong view? yes he does, but as you can see he is never " thou shalt believe in rebirth... or else!" He is much more focused on your practice here and now bringing benefits and teaches to the level of the person, from basic generosity and giving up to kamma. I think that western people today get really hung up on intellectual pondering regarding topics like rebirth when they should be practicing, here and now is the most important place and time, in this precious human birth, where we can practice dhamma and free ourselves.
As far as a quick personal addition to this if I am allowed. I am a practicing Buddhist, have been for 6 years. I'm also close to getting on the path to renunciation and becoming a monk. I cannot say I BELIEVE in rebirth and that has never been a hindrance to me in Theravada. No monk has scolded me,judged me, or held me back in any way.
I am an agnostic and keep an open mind. I have also through my practice proven the Buddha to be dead on accurate about many things, so at this point I have enough confidence and faith in the dhamma to say "hmm.. if the Buddha was right about all this.. why not that?".

A view that rebirth does not exist is a wrong view. You cannot realize nibbana with this wrong view.

Kalama sutta isn't overrated. But yes, it is often misquoted and misunderstood. People rarely mention the section which says "praised by the wise" or "criticised by the wise".  It's important not to take the Sutta out of context and think that this is the approach Buddha would recommend to all. Kalama was not already a Buddhist when the Buddha preached the sutta to him. He was a confused person and didn't know what teaching to follow. But once you take refuge in the Triple gem, you should take the things you don't understand by faith until you are able to understand through direct knowledge.

No, believe in literal rebirth is not necessary, as long as you don't subscribe to the other extreme -- that of complete annihilation at the time of death.
That said, Buddha greatly appreciated the concept of rebirth as a practical motivator. He compared fear of unfortunate rebirth with fear of punishment that stops a potential criminal from committing a crime.
One of my teachers (an ordained Korean Zen master) did not assert literal rebirth, stating instead that rebirth happens every moment. My other teacher explicitly said that rebirth/karma is a metaphor for information carried on from one life to another through natural means.

"Rejecting rebirth is part of wrong view". See @catpnosis answer.

This is more a question of the meaning of words than of doctrine or practice per se. If one defines "Buddhism" to include a belief in re-birth, then you can't have the former without the latter. But if not, then otherwise. The problem is, there is no precise, universally accepted definition of "Buddhism". As Jayantha points out, so-called "secular" Buddhism doesn't tend to have a belief in re-birth. Now some people would reply that in that case "secular" Buddhist's aren't actually Buddhists -- the No True Scotsman position (because, as I said, there is not a universally accepted definition), but that's just begging the question.
An analogy could be to ask if someone could be a Christian without believing in the divinity of Christ. The majority of people calling themselves Christian would say you cannot, but Christadelphians would say you can. Again, many Christians would then say, as a result, that Christadelphians are not "real" Christians. Again; question begging.
So, is it possible to meditate without believing in re-birth? Yes.
Without believing in re-birth...

...is it possible to practice and develop compassion? Yes.
...is it possible to hold the Buddha's teaching in high esteem, but to see
"re-birth" as simply a metaphor? Yes.
...is it possible to achieve a
high level of "attainment" -- advanced samadhi, highly developed
insight and so on? Yes, apparently so.

However, is a person missing something significant if re-birth is true but they don't see that and act accordingly? I'd say, absolutely.
As I result, personally I concur with @Jayantha's approach -- currently I don't know but I have an open mind. I'm also taking care to understand what the Buddhist view of "re-birth" actually is. It's widely misrepresented (as is, to go back to my analogy, the exact nature of Christ's alleged "divinity"). The more I look at Buddhism, the more I realize that the popularized ideas often vastly oversimplify what are actually very deep and subtle concepts, with far more scope for being true and consistent with my mainline western-science-based upbringing than I had at first thought when viewing them from a distance.

That's really interesting thank you. My feeling is that the divinity of Christ is more central to (most) Christians than the issue of rebirth is to (most) Buddhists. That is just a feeling though

That's true (although Christadelphians would of course say that it's not central for them :-) ). A better example might have been something like the position of Mary the mother of Jesus: the immaculate conception, the virgin birth, her assumption into heaven, and so on. My point was simply that once we wrap a big bag of concepts under one label -- "Buddhist", "Christian" -- then questions about whether one is a true or wholehearted example of the thing can be as much to do with the simple creating of that label rather than anything more substantial in terms of the underlying religion/etc.

Rejecting rebirth is part of wrong view. Rejecting rebirth hence is also an unwholesome deed. But, being 'agnostic' you don't need to reject rebirth, you may just put this belief on hold. I don't see, though, how can you understand and appreciate karma at full without belief in rebirth. If there is no rebirth it may reasonably follow that you can avoid results of unwholesome deeds if you are astute enough.
From other point of view, I think, from Abhidharmic perspective, if we strictly follow dharmical analytical view, then belief in rebirth is not that important, because in that perspective 'being' and 'death' are not real existents and just conventional truth, there will be just indefinite long stream of consciousness (santana) where karma fruits will necessarily appear for every action (cetana). From the same perspective we can presume that we are already rebirthing every moment.

Belief in anything does not make it true. Doubt in anything does not make not true. In the first part of your answer, your "logic" is simply "lawyeristic jingoism". However, I definitely do like the second paragraph of your answer. To quote your answer, "'being' and 'death' are not real existents and just conventional truth". Bingo. Indeed, notice that your awareness arises every time you wake up. After seing that, you may begin to see that life just seems to keep getting weirder and weirder, beyond analyzation. Enjoy!

it is wrong view to deny fruits of karma or that states of 'being' are 'spontaneously/immediately born' from acts of (self-view based) karma. However, to equate the loose translation 'rebirth' with 'reincarnation' or 'post-mortem-rebirth' has little basis in experiencable reality or in the suttas (apart from a few literal reincarnation suttas that were obviously placed into the suttas at a later date). There is little evidence 'rebirth' equates with an afterlife when the definitions of 'birth', 'death' & 'body' ('kaya') are understood. Regards

@Dhammadhatu Nowhere I equated 'rebirth' with 'reincarnation'. Aren't we say 'rebirth' to deliberately distinguish it from reincarnation?

I said 'post-mortem'. To believe 'rebirth' occurs after after physical death is  'reincarnation', which is why Tibetans & the Ajahn Brahm cult teach about literal reincarnation. Buddha defined the word 'kaya' as 'the collection of the five aggregates' & defined 'death' as the death of the idea of 'a being', 'person' or 'self' ('satta'). If those Pali teachings about karma reaped 'after death' were about a next life then no karma (example a robber going to prison) would not be reaped in this life. Thus this mundane right view of karmic results does not necessarity relate to another life.

@Dhammadhatu 1) Word 'rebirth' is used commonly to signify next lives and to be distinguished from non-buddhist 'reincarnation' views. I'm just used standard term, and I din't invented it. I didn't meant reincarnation. Thus, reincarnations criticisms aren't applicable to my answer. 2) Your concern about if 'wrong view' is denying next births is valid.

@Dhammadhatu I posted new question regarding your concerns: http://buddhism.stackexchange.com/questions/15966/does-natthika-dithi-is-actually-denying-rebirths

So in short is a belief in rebirth a necessity for Buddhist practice?

After going through all the responses above I was very much satisfied with what Jayantha has said.
I would like to add something to this discussion -
I think rebirth is part of Buddhist doctrine. But the real question is rebirth of what? and rebirth of whom?
According to the Buddha, there are four elements of Existence which go to compose the body. They are (1) Prithvi; (2) Apa; (3) Tej; and (4) Vayu. Question is, when the human body dies, what happens to these four elements? Do they also die along with dead body? According to The Buddha they join the mass of similar elements floating in (Akash) space. When the four elements from this floating mass join together, a new birth takes place. This is what the Buddha meant by rebirth, as I have understood it. It must be noted that the body dies. But the elements are ever-living. This is the kind of rebirth in which the Buddha believed. Great light is thrown upon the subject by Sariputta in his dialogue with Maha-Kotthita. The Buddha was not an absolute annihilationist. He was an annihilationist so far as soul was concerned. He was not an annihilationist so far as matter was concerned. (I think Andrei Volkov is also saying the same point when he is talking about "complete annihilation).
There is another concern here. It is about rebirth of Whom? Does the same dead person take a new birth? Did the Buddha believe in this thesis? The answer is, "Most improbable." The answer depends upon the elements of existence of the dead man meeting together and forming a new body; then the possibility of the rebirth of the same sentient being is possible. If a new body is formed after a mixture or the different elements of the different men who are dead, then there is rebirth but not the rebirth of the same sentient being. This point has been well explained by sister Khema to King Pasenadi. Where she ends up saying 'The Tathagata exists after death...exists not after death', does not apply." (Again we can match Jayantha's write up regarding Kalama Sutta.)
I have drawn lot from Buddha and His Dhamma Book 4, Part 2, section 1, sub section 2. Hoping for further discussion.

Most of the answers say "No" but I would like to offer a different point of view eventhough it will not be popular, sorry!
The key concept of Buddhism are the 4 Noble truths. They are shared by all schools and were part of the first discourse of the Buddha, the 4 Noble truths are so important that they keep appearing over and over again in many suttas.
I cannot see how can one be a Buddhist disagreeing our doubting the 4 Noble truths. So, if you check them you will see that is not possible to agree with them and at the same time disagree with the idea ofthings like samsara, rebirth, etc
The first thing Buddha said after enlightment (is in the dhammapada) was: " I have gone round in vain the cycles of many lives ever striving to find the builder of the house of life and death. How great is the sorrow of life that must die!"
The 1st Noble truth is not about ordinary suffering only, but mostly about ageing, sickness and death (and rebirth! and lower realms!)
Scaping from Samsara by achieving Nibbana is also part of the noble truths... so if you think about it carefully you may see that: Yes, it is necessary to believe in those things to accept the Noble truths and without the Noble truths I cannot see how can one be a Buddhist (can be a nice person for sure, but not a followr of the Buddha)
Buddha said this verse in Dhammapada 176 to show the importance of it: "For a liar who has violated the one law (of truthfulness) who holds in scorn the hereafter, there is no evil that he cannot do."

According to the Buddhism, "Next Birth" is not the person who lives in this birth. In the next birth, there is a person determined by the previous Karma.
For certain concepts to be describe in particular in anicca it is needed to believe reincarnation.
All of what we are ends with this birth. Only goodness and evil are taken to the next birth. So there is need to believe it for this reason.

For instance, If we are having good wealth and satisfaction through
  all the ways, we may think life is so happy and funny. But it is due
  to the meritorial deeds you have done in your past births. The effects
  of those good merits will end in one day. At the end of this birth or
  in future births. No one will be a good person all over his Sansara.
  So one day comes where you have to encounter sadness.

From here:

§ When I first went to practice meditation with Ajaan Fuang, I asked
  him if people really were reborn after death. He answered, "When you
  start out practicing, the Buddha asks you to believe in only one
  thing: karma. As for things aside from that, whether or not you
  believe them isn't really important."

You may find the Apannaka Sutta MN 60 (A Safe Bet) helpful to answer your question. Here the Buddha talks about the benefits of not rejecting the possibility of rebirth.
"As you have not found an agreeable teacher, you should adopt and practice this safe-bet teaching, for this safe-bet teaching — when accepted and adopted — will be to your long-term welfare & happiness.
"And what is the safe-bet teaching?
http://www.accesstoinsight.org/tipitaka/mn/mn.060.than.html

Chong An Sunim talked about this in one dharma talk by saying how people ask him if rebirth exist or not. He said he replies, "what is it that you want? If you want rebirth, you have it. If you don't want it, you don't have it."
Rebirth is just a teaching word. It is there only to give you some intellectual understand of what is going on. In Zen, finger is pointing to moon. There is no need to argue whether finger is worth belief or not. Use it as tool and look at the moon.
Don't use rebirth as belief. Don't use it as non-belief. Use it as stepping stone. Use it as aid. Buddhism is about practice. You can only truly understand meaning and ultimate reality behind this concept by seeing it clearly as it is. And that is not depended upon whether you believe in it or not. Both of those views can be hindrance in seeing how reality really is.
Thus belief in rebirth or reject of rebirth are both wrong views. Any kind of idea you have about rebirth is just idea and in Buddhism all ideas are wrong ideas. Don't believe. Don't reject. Just use it as instruction manual. Aim to see clearly and past all of these concepts because ultimate reality does not support existence of these concepts.
Instead of existence of rebirth it could be more beneficial to meditate and contemplate the definition of rebirth. What is being reborn? When? How?

Yes.
View of rebirth is a part of Right View, but only at the later stages, after one sees for oneself that rebirth is real.
Read The Noble Eightfold Path by Bhikkhu Bodhi.


text:
What are the Five Precepts?
And where are they found in the Pali Canon? I realize that this is a Straw Man question for the group but I would like to raise our visibility on  the Web and perhaps start our community wiki.

It is the start of Abhisanda Sutta (AN) 8:39:

"Now, there are these five gifts, five great gifts — original, long-standing, traditional, ancient, unadulterated, unadulterated from the beginning — that are not open to suspicion, will never be open to suspicion, and are unfaulted by knowledgeable contemplatives & brahmans. Which five?
"There is the case where a disciple of the noble ones, abandoning the taking of life, abstains from taking life.  In doing so, he gives freedom from danger, freedom from animosity, freedom from oppression to limitless numbers of beings. In giving freedom from danger, freedom from animosity, freedom from oppression to limitless numbers of beings, he gains a share in limitless freedom from danger, freedom from animosity, and freedom from oppression. […]
"Furthermore, abandoning taking what is not given (stealing), the disciple of the noble ones abstains from taking what is not given. In doing so, he gives freedom from danger, freedom from animosity, freedom from oppression to limitless numbers of beings. […]
"Furthermore, abandoning illicit sex, the disciple of the noble ones abstains from illicit sex. In doing so, he gives freedom from danger, freedom from animosity, freedom from oppression to limitless numbers of beings […]
"Furthermore, abandoning lying, the disciple of the noble ones abstains from lying. In doing so, he gives freedom from danger, freedom from animosity, freedom from oppression to limitless numbers of beings. […]
"Furthermore, abandoning the use of intoxicants, the disciple of the noble ones abstains from taking intoxicants. In doing so, he gives freedom from danger, freedom from animosity, freedom from oppression to limitless numbers of beings. […]

These are the "five precepts" or pañca-sikkhāpada. They are frequently stated in English as:

I undertake the training rule to abstain from killing.
I undertake the training rule to abstain from taking what is not given.
I undertake the training rule to avoid sexual misconduct.
I undertake the training rule to abstain from false speech.
I undertake the training rule to abstain from fermented drink that causes heedlessness.

Which you can see along with the Eight Precepts at The Five Precepts

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What is the relevance of Pratityasamutpada in Nagarjunas system?
In his philosophy, Nagarjuna uses the Pratityasamutpada to prove, that everything is empty and without reality. In the end, however he proves not only the emptiness of that very Pratityasamutpada but he also somehow leaves the law (of dependent arising) intact and working. Does anyone understand his point, especially in what concerns Pratityasamutpada?

Emptiness is not denial. It's special technical term for absence of something while presence of another. Like empty bottle isn't non-existent bottle, it's just bottle w/o content.

Where do I begin? ;)

@catpnosis A favourite mahayanic/madhyamic simile says that so called old buddhism (=hinayana) talks of empty bottles while the mahayana proves that there is no bottle...

@zvolkov Anywhere!

The real point is that There Is No Spoon. Forget the bottle.

I took a university course in Asian Philosophy and my professor explained emptiness this way:

Nagarjuna makes the claim that an essence (a list of essential qualities that make a thing what it is) can only be stated in reference to other things. This is a way of saying that all things are defined in relation to other things, including samsara and nirvana.
The point of doing this, supposedly, is to help us realize that this means enlightenment and samsara are defined in terms of each other: Without samsara we can't define enlightenment, and without enlightenment we can't define samsara.

Basically according to this professor, Nagarjuna makes the argument that "We're all in this together" and we should be helping each other reach enlightenment and not strive for it by ourselves.
He also stated that Emptiness is a flashy re-packaging of the teaching of dependent origination.
In my opinion, the concept of Emptiness makes sense, but I don't really see how it's useful above and beyond the teachings in Early Buddhism. If anything, an enlightened person is more able to help others, so there shouldn't be a problem in striving towards enlightenment.

To me it seems, that the concept of emptiness is just some sort of philosophical support for the Buddha's refusal to teach metaphysics, in that it demolishes all (philosophical) speculation.

Nagarjuna uses Pratityasamutpada as a proof for emptiness by means of his many arguments found in chapter 1 of the Mulamadhyamakakarika. Basically he lays out a series of reductio ad absurdum arguments that show that if you posit real phenomena, it absurdly follows that change and arising cannon occur. But because Pratityasamutpada does occur, the initial assumption that phenomena are real is refuted.
Later in the work (i.e. Chapter 24) he says basically the same thing but the other way around. He addresses the possible criticism that emptiness negates core Buddhist teachings, and Nagarjuna argues that because of the fact that if emptiness were false, change could not occur, emptiness doesn't negate core Buddhist teachings, but is in fact the only thing that makes them possible in the first place.
Putting these together we conclude that Pratityasamutpada proves emptiness, and emptiness makes Pratityasamutpada possible. Pratityasamutpada explains the conventional level of things, and emptiness explains them on the ultimate level, and these two fit together in the union of appearance and emptiness, and Nagarjuna famously declares:

Whatever is dependently co-arisen
That is explained to be emptiness.
That, being a dependent designation
Is itself the middle way.
XXIV.18

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text:
What concordances are there for the Pali Canon?
I am amazed at how quickly folks on this SE find detailed quotes from the canon.  Are folks using a concordance?  (That's what Christians call it for the Bible--it's a dictionary of topics cross-referenced to the source text so that it's easier to look up passages by subject.)
If there is a concordance for the Pali Canon, where can it be found?

I had to look up "concordance" in the dictionary. Do you mean a list of words? If so, there is one here: http://www.accesstoinsight.org/index-subject.html

Make that an answer and I'll vote it up. :-)

Google search also helps.

Google searches aren't curated.  Lots of quantity, little quality. :-)

Here's a list of terms in the Pali Canon that can be used to quickly find particular suttas:
http://www.accesstoinsight.org/index-subject.html

The number of suttas available here is quite limited. And the translations are idiosyncratic. So this is not the best resource available. Better to use one of the electronic versions.

There is the: Pali Tipitakam Concordance: Being a Concordance in Pali to the Three Baskets of Buddhist Sciptures in the Indian order of letters, listed by F.L. Woodward.
3 Volumes
Available from the Pali Text Society:
http://palitext.com/
Not much use if you are not familiar with and have the Pali.
A concordance is a work giving the page locations of a list of words or words in passages of a given work or set of works.
A good way to quickly find passages you are looking to research is to use the Pali English Dictionary. (also available from the PTS, and on line at:
http://dsal.uchicago.edu/dictionaries/pali/index.html )
The problem with both of these is the need to look things up using the Pali ordering of the letters.
Another way people are using is to look up key words in the CSCD which will give the Sutta Citation which can then be looked up in the various translations available.
Obtain the CSCD from
the Vipassana Research Institute
http://www.vridhamma.org/Chattha-Sangayana-CD-ROM-Update

I also use the VRI Burmese Ed. It is far better to have electronic search than a concordance.


text:
What are the Four Noble Truths?
What are they?  Where are they found in the literature?  Are there any significant differences in them among the traditions?

Here is a version from the Triratna site

All existence is dukkha
The cause of dukkha is craving
The cessation of dukkha comes with the cessation of craving
There is a path that leads from dukkha - (The path most commonly associated with this is the eightfold path).

Dukkha is commonly translated as suffering but the translation that I find more useful is unsatisfactoriness. The original meaning comes from an ill fitting chariot wheel. A bumpy ride if you will.
The formulation is related to a medical diagnosis e.g.

The illness
The etiology (cause)
The prognosis
The cure

As far as I am aware the 4 Noble Truths are a central teaching in Buddhism perhaps comparable to the divinity of Christ for Christian's (controversial comparison maybe). I have not heard of any significant variations. I would be very interested to hear if they were any.

 1. What are they?
They are the diagnosis, description, prognosis & prescription for emotional suffering, distress and dissatisfaction.
2. Where are they found in the literature?
The most succinct & direct description is found in the Discourse on Setting the Wheel of Dhamma in Motion (SN 56.11, Dhammacakkappavattana-suttaṃ).
However, the most detailed description is found in one of the sections of the Greater Discourse on Establishing Mindfulness (DN 22, Mahāsatipaṭṭhāna-suttaṃ) & the Discourse on the Dissection of the Truths (MN 141, Saccavibhaṅga-suttaṃ).
Even though these discourses are the most explicit, the four truths are found throughout the whole Pāḷi Canon.
3. Are there any significant differences in them among the traditions?
According to Thich Nhat Hanh, craving as a cause for suffering is just an abbreviation for a list of causes, craving being the first cause in the list:

"Another common misunderstanding of the Buddha's teaching is that all of our suffering is caused by craving. In the Discourse on Turning the Wheel of the Dharma, the Buddha did say that craving is the cause of suffering, but he said this because craving is the first on the list of afflictions (kleshas). If we use our intelligence, we can see that craving can be a cause of pain, but other afflictions such as anger, ignorance, suspicion, arrogance, and wrong views can also cause pain and suffering. Ignorance, which gives rise to wrong perceptions, is responsible for much of our pain. To make the sutras shorter and therefore easier to memorize, the first item on a list was often used to represent the whole list. The word "eyes," for example, is used in many sutras to represent all six sense organs and "form" is often used to represent all Five Aggregates (skandhas). If we practice identifying the causes of our suffering, we will see that sometimes it is due to craving and sometimes it is due to other factors."
The Heart of the Buddha's Teachings, Thich Nhat Hanh. Ch. 5: Is Everything Suffering?

sunyata, interesting edit. I've tried to make the wording more direct. Is it possible to provide a quote from the book so that we can read Thich Nhat Hanh's words on the subject?

@Anthony: I've added a link to a quote from TNH and i just updated it with a longer quote which includes the comparison with writing down "eyes" for all six senses. Is this ok?

@sunyata it's ok but i've tried to bring it closer to the StackExchange model by incorporating the text into the answer as a quote.

@Anthony: Thank you, much better after your edit. Slightly off topic but connected: This same chapter in "The Heart of the Buddha's Teachings" contains other (in the author's view) misunderstandings of the Buddha's teachings (maybe this is a discussion for meta?)

Here's the Dhammacakkapavattana Sutta (SN 56.11), supposedly the earliest occurence of the FNT in the Canon:

[beginning skipped]
"Now this, monks, is the noble truth of stress:1 Birth is stressful, aging is stressful, death is stressful; sorrow, lamentation, pain, distress, & despair are stressful; association with the unbeloved is stressful, separation from the loved is stressful, not getting what is wanted is stressful. In short, the five clinging-aggregates are stressful.
"And this, monks, is the noble truth of the origination of stress: the craving that makes for further becoming — accompanied by passion & delight, relishing now here & now there — i.e., craving for sensual pleasure, craving for becoming, craving for non-becoming.
"And this, monks, is the noble truth of the cessation of stress: the remainderless fading & cessation, renunciation, relinquishment, release, & letting go of that very craving.
"And this, monks, is the noble truth of the way of practice leading to the cessation of stress: precisely this Noble Eightfold Path — right view, right resolve, right speech, right action, right livelihood, right effort, right mindfulness, right concentration.
[ending skipped]

"The Four Truths of The Nobles" are a formulaic summary of Buddha's Awakening. They are easier understood in context of Three Characteristics of Existence:

Whatever is compound (consists of multiple pieces coming together), will fall apart. Same way, whatever is conditional (depends on multiple factors coinciding together), is impermanent.
Nothing is solid, everything is made of pieces or depends on conditions. This includes "I" which cannot possibly be a solid/independent entity. Since "I" is really just a compound phenomena, it is not independent and too is subject to conditions.
Because of the above, there is no (and there can't be!) such thing as permanent Sukha ("ease", "comfort") . Dukkha ("wrongness", "trouble") is an inevitable part of existence at large.

Now, instead of being a helpless victim caught in this universal trap, let us take master's stance, as was Buddha's habit:
Whatever comes together, can be broken apart. Therefore, whatever is conditional (depends on multiple factors coming together), can be controlled. Because all subjective experiences are conditional, they can all be controlled. This includes experience of Dukkha ("wrongness", "trouble"), which is a conditional phenomena and therefore can be controlled. All we need is 1) analyze Dukkha 2) identify factors comprising Dukkha 3) pick a factor in our control that we can remove, thereby disbanding the entire compound Dukkha 4) work towards removing that factor.

There is Dukkha, this life is not without Dukkha ("wrongness", "trouble"). Dukkha must be understood. Dukkha comes from superposition of "that" and "this". "That" is a desirable possibility, object of unsatisfied thirst. When a desirable "that" emerges in the mind, "this" co-emerges as undesirable, Dukkha. From emergence of "that" comes emergence of "this". When "that" does not exist, "this" does not exist. From cessation of "that" comes cessation of "this". Dukkha has been understood.
Dukkha depends on factors, is not without its factors. The key factor of Dukkha is "Thirst" -- unrealistic clinging to "that", or irrational attachment to "this". The Thirst must be let go. The Thirst has been let go.
Dukkha is subject to disbanding of its factors, is not immune to disbanding of its factors. With disbanding of factors of Dukkha comes cessation of Dukkha. Cessation of Dukkha must be realized. Cessation of Dukkha has been realized.
There is Path to cessation of Dukkha, cessation of Dukkha is not without a path. Path must be actualized. Path has been actualized.

May I suggest you make more explicit that the Path is the Noble Eightfold Path?

I left that out on purpose, because according to some research the Eightfold Path as we know it today could be a later compilation that did not exist at Buddha's time. Buddha did mention Ancient Path but not necessarily all of its components at once.

Fascinating!  If it's not too much trouble could you add your findings to the N8P question?

According to the Saccavibhanga Sutta translated by
Thanissaro Bhikkhu:

the noble truth of stress.

Now what, friends, is the noble truth of stress? Birth is stressful, aging is stressful, death is stressful; sorrow, lamentation, pain, distress, & despair are stressful; not getting what is wanted is stressful. In short, the five clinging-aggregates are stressful.

And what are the five clinging-aggregates that, in short, are stressful? The clinging-aggregate of form, the clinging-aggregate of feeling, the clinging-aggregate of perception, the clinging-aggregate of fabrications, the clinging-aggregate of consciousness: These are called the five clinging-aggregates that, in short, are stressful.

the noble truth of the origination of stress.

And what, friends, is the noble truth of the origination of stress? The craving that makes for further becoming — accompanied by passion & delight, relishing now here & now there — i.e., craving for sensuality, craving for becoming, craving for non-becoming.

the noble truth of the cessation of stress.

And what, friends, is the noble truth of the cessation of stress? The remainderless fading & cessation, renunciation, relinquishment, release, & letting go of that very craving.

the noble truth of the path of practice leading to the cessation of stress.

And what, friends, is the noble truth of the path of practice leading to the cessation of stress? Just this very noble eightfold path: right view, right resolve, right speech, right action, right livelihood, right effort, right mindfulness, right concentration.

According to the Tittha Sutta, translated by Thanissaro Bhikkhu, the second and third noble truths are related to the Twelve Links of Dependent Origination.

And what is the noble truth of the origination of stress? [the second noble truth]

From ignorance as a requisite condition come fabrications. From fabrications as a requisite condition comes consciousness. From consciousness as a requisite condition comes name-&-form. From name-&-form as a requisite condition come the six sense media. From the six sense media as a requisite condition comes contact. From contact as a requisite condition comes feeling. From feeling as a requisite condition comes craving. From craving as a requisite condition comes clinging/sustenance. From clinging/sustenance as a requisite condition comes becoming. From becoming as a requisite condition comes birth. From birth as a requisite condition, then old age & death, sorrow, lamentation, pain, distress, & despair come into play. Such is the origination of this entire mass of stress & suffering.

And what is the noble truth of the cessation of stress? [the third noble truth]

From the remainderless fading & cessation of that very ignorance comes the cessation of fabrications. From the cessation of fabrications comes the cessation of consciousness. From the cessation of consciousness comes the cessation of name-&-form. From the cessation of name-&-form comes the cessation of the six sense media. From the cessation of the six sense media comes the cessation of contact. From the cessation of contact comes the cessation of feeling. From the cessation of feeling comes the cessation of craving. From the cessation of craving comes the cessation of clinging/sustenance. From the cessation of clinging/sustenance comes the cessation of becoming. From the cessation of becoming comes the cessation of birth. From the cessation of birth, then old age & death, sorrow, lamentation, pain, distress, & despair all cease. Such is the cessation of this entire mass of stress & suffering.

The way I explain the Four Noble Truths to non-Buddhists is:
The Four Noble Truths

Life means suffering
The origin of suffering is attachment
It is possible to end suffering through...
...The Eightfold Path


text:
What is the Noble Eightfold Path?
Where is the Noble Eightfold Path found in the Pali Canon?  Are there significant differences among the traditions in its interpretation?

The way I explain The Path to non-Buddhists is:
The Noble Eightfold Path

Right View (The Four Noble Truths)
Right Intention

Renunciation:
Resistance to desire
Goodwill:
Resistance to anger and aversion
Harmlessness:
Compassion,
Don't think or act cruelly, violently or aggressively

Right Speech

Tell the truth
Don't gossip
Don't use offensive or hurtful language

Right Action (The Precepts)

Don't harm other living beings
Don't take things not freely given
Don't engage in sexual misconduct
Don't engage in false speech
Don't abuse drugs or alcohol

Right Livelihood
Don't work in a job that violates The Precepts
Right Effort

Create, preserve and increase healthy states
Prevent, eliminate or decrease harmful states

Right Mindfulness

See things clearly
The Four Foundations of Mindfulness
Be mindful of:

body
feeling
state of mind
phenomena

Right Concentration

Meditation
Concentration on healthy thoughts and actions

According to Magga-vibhanga Sutta: An Analysis of the Path
Now what, is the Noble Eightfold Path?

Right view

Knowledge with regard to stress, knowledge with regard to the origination of stress, knowledge with regard to the stopping of stress, knowledge with regard to the way of practice leading to the stopping of stress

Right resolve

Being resolved on renunciation, on freedom from ill will, on harmlessness

Right speech

Abstaining from lying, abstaining from divisive speech, abstaining from abusive speech, abstaining from idle chatter

Right action

Abstaining from taking life, abstaining from stealing, abstaining from unchastity

Right livelihood

There is the case where a disciple of the noble ones, having abandoned dishonest livelihood, keeps his life going with right livelihood

Right effort

(i) There is the case where a monk generates desire, endeavors, activates persistence, upholds & exerts his intent for the sake of the non-arising of evil, unskillful qualities that have not yet arisen.
(ii)He generates desire, endeavors, activates persistence, upholds & exerts his intent for the sake of the abandonment of evil, unskillful qualities that have arisen.
(iii)He generates desire, endeavors, activates persistence, upholds & exerts his intent for the sake of the arising of skillful qualities that have not yet arisen.
(iv)He generates desire, endeavors, activates persistence, upholds & exerts his intent for the maintenance, non-confusion, increase, plenitude, development, & culmination of skillful qualities that have arisen

Right mindfulness

(i) There is the case where a monk remains focused on the body in & of itself — ardent, aware, & mindful — putting away greed & distress with reference to the world.
(ii) He remains focused on feelings in & of themselves — ardent, aware, & mindful — putting away greed & distress with reference to the world.
(iii) He remains focused on the mind in & of itself — ardent, aware, & mindful — putting away greed & distress with reference to the world.
(iv) He remains focused on mental qualities in & of themselves — ardent, aware, & mindful — putting away greed & distress with reference to the world.

Right concentration

(i) There is the case where a monk — quite withdrawn from sensuality, withdrawn from unskillful (mental) qualities — enters & remains in the first jhana: rapture & pleasure born from withdrawal, accompanied by directed thought & evaluation.
(ii) With the stilling of directed thoughts & evaluations, he enters & remains in the second jhana: rapture & pleasure born of concentration, unification of awareness free from directed thought & evaluation — internal assurance.
(iii) With the fading of rapture, he remains equanimous, mindful, & alert, and senses pleasure with the body. He enters & remains in the third jhana, of which the Noble Ones declare, 'Equanimous & mindful, he has a pleasant abiding.'
(iv) With the abandoning of pleasure & pain — as with the earlier disappearance of elation & distress — he enters & remains in the fourth jhana: purity of equanimity & mindfulness, neither pleasure nor pain.

1. What is the Noble Eightfold Path?
The Noble Eightfold Path is the prescription, made known by the Buddha, for emotional suffering, distress & dissatisfaction.

The Buddha: I saw an ancient path, an ancient road, traveled by the Rightly Self-awakened Ones of former times. And what is that ancient path, that ancient road, traveled by the Rightly Self-awakened Ones of former times? Just this noble eightfold path: right view, right aspiration, right speech, right action, right livelihood, right effort, right mindfulness, right concentration. That is the ancient path, the ancient road, traveled by the Rightly Self-awakened Ones of former times. I followed that path. Following it, I came to direct knowledge of aging & death, direct knowledge of the origination of aging & death, direct knowledge of the cessation of aging & death, direct knowledge of the path leading to the cessation of aging & death. I followed that path. Following it, I came to direct knowledge of birth... becoming... clinging... craving... feeling... contact... the six sense media... name-&-form... consciousness, direct knowledge of the origination of consciousness, direct knowledge of the cessation of consciousness, direct knowledge of the path leading to the cessation of consciousness. I followed that path.
-SN 12.65, Nagara-suttaṃ

(For details about what constitutes the path see the reference provided in the next answer.)
2. Where is the Noble Eightfold Path found in the Pali Canon?
It is described in many places throughout the Canon but one detailed example is the Discourse on the Dissection of the Path (SN 45.8, Maggavibhaṅga-suttaṃ).
3. Are there significant differences among the traditions in its interpretation?
No, but as a related side-note, the path culminates with two additional stages, Right Knowledge & Right Release, making it tenfold, as is stated in MN 78, Samaṇamuṇḍika-suttaṃ:

Now, an individual endowed with which ten qualities is one whom I describe as being consummate in what is skillful, foremost in what is skillful, an invincible contemplative attained to the highest attainments? One endowed with the right view of one beyond training, the right resolve of one beyond training, the right speech... the right action... the right livelihood... the right effort... the right mindfulness... the right concentration... the right knowledge... the right release of one beyond training. An individual endowed with these ten qualities is one whom I describe as being consummate in what is skillful, foremost in what is skillful, an invincible contemplative attained to the highest attainments.

May I suggest that you add the relevant passages in block quotes to supplement your links?  All things are transient, links even more so. :-)

A whole discourse might be a bit excessive but I can add this last reference since it's just a paragraph.

Let us try to understand the word "samma". It has 03 meanings. "Yatavata, Niyanika and Kusala".
01. Yatavata means Absolutely correct,
02. Niyanika means definitely satisfying [by reaching Nirvana].
03. Kusala means merit[Definitely destroys evil],

Then because it does not change, because it leads to nirvana, we say merit by merit is "samma".
Good vision, that is, "samma ditti" is something that does not change.
A remedy, a course of action. If someone has samma ditti, he will definitely attain nirvana. If it is endowed with samma ditti, that mind will surely become meritorious. Moreover, the Samma Ditti is divided according to merit consciousness.

The worldly["Laukikika"] samma ditti - Worldly samma ditti is the samma ditti that arises with meritorious minds in this worldly lands like Kama vachara[Earthly world, 6 heavens], Rupavachara[16- Brahma Loka], Arupa vachara[4- Brahma Loka] etc.
The transcendental["Lokoththara"] samma ditti.- Transcendental samma ditti is the samma ditti that occurs during the fruition of the transcendental path.

Under the worldly samma ditti, in several ways it is explained namely
**01.**Kammassakata samma ditti, -  means the samma ditti of believing in karma and the result of karma. Now look, those who commit deadly evil will definitely be born in the four hells, and those who live by stealing will be born in hell. Likewise, people who misbehave in lust, and those who live by telling lies, will be born in hell. People who are addicted to gambling are born in hell.
Also, if someone steel the things belongs to the Lord Buddha and the Sangha[Monks] or dharma are born in Avicii great evil, those who live by killing mother, father, shaking the blood of Buddha, creating sectarian divisions among sangha[Monks] are born in Avicii great evil, those who live by killing goats, chickens, pigs, are born in Sanjiva evil, in that way, according to their badness of karma Lord Buddha preached.
Likewise, those who do charity are born in the divine world or the human world. Those who practice meditation are born in the Brahma worlds. We call the samma ditti of believing about this karma as 'kammassakata samma ditti'.
**02.**Dasa Vatthuka samma ditti - If someone think with the below 10 types of faith, then it is called "Dasa Vaththuka Samma Ditti".
Those are
 1. That there are rewards for giving
 2. That there are rewards for sacrifices
 3. That there are rewards for sending gifts to the virtuous
 4. That there are rewards for doing good and bad deeds,
 5. That those who are in the hereafter will return to this world as births,
 6. That those who are in this world will come to the other world as births
 7. That there are consequences for the good and bad deeds done to the mother,
 8. That there are consequences for the good and bad deeds done to the father,
 9. That there are beings who are reborn after death,
 10. That there are beings who are reborn after death, and that there are
     Brahmins in the world who follow the good path and follow
     the good practices of this world and the hereafter, who perceive the
     world and the hereafter with their wisdom.

**03.**Jhana samma ditti - During meditation, using "samatha"  meditation "karmasthanas"[40 Samatha meditation methods] and achieve  meditation, samadhi, samapati, and abhijna in the meditation, the samadhi that occurs in the mind, and the wisdom that occurs with the samadhi mind is called Jhana samma ditti
04.****Vipassana samma ditti- Vidarshana meditation means that we  investigate our body using  our consciousness whether it is made of gold, silver, pearls, gems etc.? or else Eye hairs, fur nails, teeth etc. These parts are broken and damaged and moving towards destruction. All these things are impermanently, painfully, inanimately meditated upon, Vidarshana. The samma ditti that occurs in Vidarshana meditation is called Vidarshana samma ditti.
**05.**Transcendentally, Chatusachchasammaditti[Four Noble Truth Samma Ditti] -, the samma ditti that occurs in the understanding of the four truths. It means that when the understanding of the

truth of suffering arises
the cause of suffering when understanding Samuda
when understanding about Nirvana,
the path to Nirvana, when understanding the path of Seela, Samadhi, and Pragna arise in the mind it is called, Chatusachcha Samma Ditti.

06.Magga samma ditti - in Buddhism there are four stages of awakening and four progressive stages culminating in full awakening (Bodhi) as an Arahant.
These four stages are

Sotāpanna,
Sakadāgāmi,
Anāgāmi, and
Arahant.

Buddha refers to people who are at one of these four stages as noble people (ariya-puggala) and the community of such persons as the noble sangha (ariya-sangha).
The 4 Magga cittas(path thoughts) and the 4 Phala cittas(fruition thoughts) can be explained as follows-
01. When magga citta occurs for the first time, it is followed by sovan phala citta.
02. When magga citta occurs for the second time, it is followed by sakadagami phala citta.
03. When magga citta occurs for the third time, it is followed by anagami phala citta.
04. When magga citta occurs for the fourth time, it is followed by arahatta phala citta.

Having explained those, now "Magga samma ditti" means the samma ditti knowledge that occurs in "Sovan marga [Sovan path]. it's magga samma ditti, Same as Sakadagami Marga gnānaya, in Anagami Marga gnānaya, in Arihat Marga gnānaya, those Samma Ditti's are called Magga samma Ditthi. Phala[Result ] samma ditti is the samma ditti that occurs in the result  knowledges of sovan phala, sakadagami phala, anagami phala, and  arihat phala.
Samma ditti can be divided and explained like this. The Samma Ditti, which was the first element of the Arya Ashtangika path[Eight fold path], which is called the "Dukkha Nirodha gamini Pratipada Arya Satya[Fourth Noble Truth]", is not the Samma Ditti of this decade. It is called Chatussatya Samma Ditti. It is a much higher Samma Ditti than this ten-object Samma Ditti.
Those who want to get rid of the sufferings of the world and come to nirvana should first of all be in the Samma Ditti of the Sasanika Dasha Vastu. Those who come to Buddhism are first established in Tisarana, and make them to follow Tisarana   in order to establish them in the Samma Ditti  of this Sasanika Dasha Vastu.
If one takes refuge in the triple gems and descends into the Samma Ditti of the Sasanika Dasha Vastu[ten objects], it is the first step towards nirvana gamini path.
There is no one who has attained nirvana by adopting a path that is not based on the Samma Ditti of the Sasanika Dasha Vastu[ten objects]. If a person got rid of the sufferings of the previous world and reached Nirvana, he reached Nirvana from all of them by staying in the Samma Ditti of the first decade and fulfilling the practices.
Similarly, you need to understand Kammassakatha Samma Ditti talking about the cause and the effects of each actions and thoughts.
Now, hope you understood how important the word Samma is.
"Arya Ashtangika Marga"
Let's talk about lord Buddha's Great Eight fold Path called "Arya Ashtangika Marga".  In simple words it has explained in all the above articles. hence would like to discuss the important divisions and word meanings as you may get deep knowledge of the eight-fold path.
Do you remember we learned three words at the beginning of this article

Yatavata means Absolutely correct,
Niyanika means definitely satisfying [by reaching Nirvana].
Kusala means merit[Definitely destroys evil],

**01.**Yatavata, Niyanika, Kusala Samma Ditti[Right Seeing]- Absolutely correct, definitely satisfying, merit Samma ditti- This is the "Pragna Chethasika[Wisdom Mental Factor] in Abhidhamma.
Samma Ditti - The Path of Suffering, Not knowing The Fourfold Truth Ignorance. It is known only as Samma Ditti. Abhidharma system mentions 8 things. Suffering, Samudaya, Nirodha, Marga, Purvantaya, Aparanta, Purvaparantaya, Paticha Samuppadaya, knowing here is samma ditti.
**02.**Yatavata, Niyanika, Kusala Samma Sankappa[Right Thought] - Absolutely correct, definitely satisfying, merit Samma Sankappa - Samma Sankappa includes[The 2 Illimitable factors and Adhimokka[Occasional mental Factor] 3 Chethasika's in Abhidamma mental factors. Literally it is called "Abhiropana Lakshana" A feature of Focusing on merit frequently with the determination of achieving Nibbana.
Samma Sankappa - three forms.
01.The thought of renunciation (nekkhamma) - Adhimokka[Determination of Nibbana]
02.The thought of non malevolence (avyapada)- Metta[Sympathetic Joy]
03.The thought of non harming (avihimsa)- Karuna[Compassion]

There are three kinds of akusala vitakka and They are:
01.Thought of sense pleasures (kama vitakka)
02.Thought of malevolence (vyapada vitakka)
03.Thought of harming (vihimsa vitakka )

Only By knowing the unwholesomeness thoughts we are able to apply below 5 methods to tame them and completely eradicate them. unwholesome thoughts - as defined under akusala, five methods of overcoming them are given:
01.by changing the object, [The Removal of affected Thoughts by Distracting]
02.thinking of the evil results [Thinking of bad sorrowful results occurring in samsara]
03.paying no attention [ neglecting them by way of let it go]
04.analyzing [ Vipassana through Abhidhamma through Nama Rupa]
05.suppressing [ Keep the thought surrender until it tamed and silent] By doing so bring them into Samma sankappa path

**03.**Yatavata, Niyanika, Kusala Samma Wacha[Right Speech] - Absolutely correct, definitely satisfying, merit Samma Wacha - one of the abstinence factors in Abhidamma mental factors. Literally it is called "Parigrahana Lakshana" A feature of Unifying, purifying, speaking words of lovingkindness, speaking the Dhamma, the crowd gathers around.  Making attractive, popular.
01. Refraining from lying - To speak the true words without lies
02. Refraining from gossiping - To speak the words of harmony without gossip
03. Refraining from filthy words - To speak gentle words without harsh language
04. Refraining from empty words - To speak soft words without speaking empty words

Always, speak good words that are beneficial to both society or the person who is listening and the for the purpose of words user.
**04.**Yatavata, Niyanika, Kusala Samma Kammanta[Right Action] - Absolutely correct, definitely satisfying, merit Samma Kammanta - one of the abstinence factors in Abhidamma mental factors. Literally it is called "Samuttapana Lakshana" A feature of Due to the merits performing using body, new merits are brought forth.
Samma Kammanta means
    01. Abstaining from killing,
    02. Abstaining from stealing,
    03. Abstaining from fornication,
    04. Abstaining from drinking alcohol,
    05. Abstaining from unrighteous physical activities.

Apart from that, samma kammanta is always applied when one exerts the body in some meritorious deeds, such as giving gifts, observing austerities, meditating, or doing practices.
**05.**Yatavata, Niyanika, Kusala Samma Aajiwa[Right Livelihood] - Absolutely correct, definitely satisfying, merit Samma Aajiwa - one of the abstinence factors in Abhidamma mental factors. Literally it is called "Vodhaana Lakshana" A feature of Cleanliness, Purity.
Samma Ajiva - Abstaining from below factors as livelihood,
01. Abstaining from killing animals for living,
02. Abstaining from stealing,
03. Abstaining from sexual immorality,
04. Abstaining from lying,
05. Abstaining from backbiting,
06. Abstaining from harsh words,
07. Abstaining from empty talk,
08. Abstaining from poison etc.

Apart from that, the mind that is used to earn happiness slowly with effort and righteousness is Samma Ajiva.
**06.**Yatavata, Niyanika, Kusala Samma Vaayaama[Right Effort] - Absolutely correct, definitely satisfying, merit Samma Vaayaama - one of the Occasional mental factors in Abhidamma chethasika section. Literally it is called "Paggaha Lakshana" A feature of Supporting other Dhammatha[Dhamma factors].
The nature of the mental process effects a division of right effort into four "great endeavors":
01. To prevent the arising of unarisen unwholesome states
02. To abandon unwholesome states that have already arisen
03. To arouse wholesome states that have not yet arisen
04. To maintain and perfect wholesome states already arisen

**07.**Yatavata, Niyanika, Kusala Samma Sathi[Right Mindfulness] - Absolutely correct, definitely satisfying, merit Samma Sathi - one of the Beautiful mental factors in Abhidamma chethasika section. Literally it is called "Upasthana Lakshana" A feature of Being able to raise merits frequently.
Samma sathi  explained as
01. Mindfulness of the body,
02. Mindfulness of the mind or pain,
03. Mindfulness of the mind
04. Mindfulness of the Dharma-natures in the mind

is the four-way Mindfulness of Samma Sathi.
**08.**Yatavata, Niyanika, Kusala Samma Samadhi[Right Concentration] - Absolutely correct, definitely satisfying, merit Samma Samadhi - one of the Universal mental factors in Abhidamma chethasika section. Literally it is called "Awikshepa Lakshana" A feature of Keeping the mind free from distraction.
The eighth factor of the path is right concentration, in Pali samma samadhi. Concentration represents an intensification of a mental factor present in every state of consciousness. This factor, one-pointedness of mind (Chitta Ekaggata), has the function of unifying the other mental factors in the task of cognition. It is the factor responsible for the individuating aspect of consciousness, ensuring that every citta or act of mind remains centered on its object
 - By not doing evil, virtue grows.
 - Concentration increases due to accumulation of merit.
 - Because of these two, Samatha Vipassana meditation increases,improves.
 - The mind is purified until the path of Dhyana, Adhigama Gnana is resultant.

Those who willing to follow buddhism should first established in Tisarana, and Tisarana is celebrated for them in order to establish them in the Samma Ditti of this Sasanika Dasha Vastu. If one takes refuge in the triple gems and descends into the Samma Ditti  of the ten objects, it is the first step towards nirvana gamini path.
Furthermore, there is no one who attains the path and attains nirvana without being in the Dasa Vastuka Samma Ditti[the ten objects Samma Ditti].
May Triple Gem Bless You!

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text:
What are the Four Immeasurables (aka Four Noble Abodes)?
Also known as the Four Noble Dwellings or the Buddhist Virtues or the Brahmaviharas.  Where in the Pali Canon are these described?

Pali canon is not systematized by topic, like this part only talk about that subject, so brahmaviharas could be anywhere and manywhere in it. It's mostly systematized by length of texts and other theme-less classificators.

@catpnosis except Samyutta Nikaya which is systematized by topic.

According to http://en.wikipedia.org/wiki/Brahmavihara the Four Noble Abodes are:

Loving-kindness (Pāli: mettā, Sanskrit: maitrī) towards all: the hope that a person will be well; "the wish that all sentient beings, without any exception, be happy."

Compassion (Pāli and Sanskrit: karuṇā): the hope that a person's sufferings will diminish; "the wish for all sentient beings to be free from suffering."

Empathetic joy (Pāli and Sanskrit: mudita): joy in the accomplishments of a person—oneself or another; sympathetic joy; "the wholesome attitude of rejoicing in the happiness and virtues of all sentient beings."

Equanimity (Pāli: upekkhā, Sanskrit: upekṣā): learning to accept loss and gain, good-repute and ill-repute, praise and censure, sorrow and happiness (Attha Loka Dhamma), all with detachment, equally, for oneself and for others. Equanimity is "not to distinguish between friend, enemy or stranger, but regard every sentient being as equal. It is a clear-minded tranquil state of mind—not being overpowered by delusions, mental dullness or agitation."

According to the Metta Sutta, Shākyamuni Buddha held that cultivation of the four immeasurables has the power to cause the practitioner to be reborn into a Brahma realm (Pāli: Brahmaloka)

According to: http://www.accesstoinsight.org/lib/authors/nyanaponika/wheel006.html:

These four attitudes are said to be excellent or sublime because they are the right or ideal way of conduct towards living beings (sattesu samma patipatti). They provide, in fact, the answer to all situations arising from social contact. They are the great removers of tension, the great peace-makers in social conflict, and the great healers of wounds suffered in the struggle of existence. They level social barriers, build harmonious communities, awaken slumbering magnanimity long forgotten, revive joy and hope long abandoned, and promote human brotherhood against the forces of egotism.

They are found in the Pali Canon in Digha Nikaya 13:
Source: http://www.accesstoinsight.org/lib/authors/nyanaponika/wheel006.html

The Basic Passage on the Four Sublime States from the Discourses of the Buddha
I. Here, monks, a disciple dwells pervading one direction with his heart filled with loving-kindness, likewise the second, the third, and the fourth direction; so above, below and around; he dwells pervading the entire world everywhere and equally with his heart filled with loving-kindness, abundant, grown great, measureless, free from enmity and free from distress.
II. Here, monks, a disciple dwells pervading one direction with his heart filled with compassion, likewise the second, the third and the fourth direction; so above, below and around; he dwells pervading the entire world everywhere and equally with his heart filled with compassion, abundant, grown great, measureless, free from enmity and free from distress.
III. Here, monks, a disciple dwells pervading one direction with his heart filled with sympathetic joy, likewise the second, the third and the fourth direction; so above, below and around; he dwells pervading the entire world everywhere and equally with his heart filled with sympathetic joy, abundant, grown great, measureless, free from enmity and free from distress.
IV. Here, monks, a disciple dwells pervading one direction with his heart filled with equanimity, likewise the second, the third and the fourth direction; so above, below and around; he dwells pervading the entire world everywhere and equally with his heart filled with equanimity, abundant, grown great, measureless, free from enmity and free from distress.

The four Brahmaviharas are four moods to be deliberately generated as antidotes against certain negative states of mind (longing, covetousness, ill will, malevolence, conceit, distress, enmity, negativity -- towards the world).

Loving-kindness. According to Chogyam Trungpa, as well as my current teacher, this is directed at self and is a foundation of compassion.
Compassion. An attitude of good king toward his subjects -- lack of harshness despite their shortcomings. Bodhisattva understands that people are different only because of their different backgrounds and experiences. In principle, there is nothing substantial that makes Bodhisattva "better" than others, beside the luck of having encountered, and benefited from, the genuine teaching. Even though Bodhisattva realizes Emptiness, s\he sees that other sentient beings still suffer because they assume things to be real. Bodhisattva understands that the subjective experience of each sentient being is basically the same, and therefore can relate to everybody's dukkha.
Empathetic joy. Feeling glad when seeing others succeed. Antidote for jealousy and competitiveness.
Equanimity. A mood that comes from an objective impersonal point of view.

Cultivation of Brahmaviharas culminates in the "state of Brahma" -- direct experience of unity with the world (AN 4.190).

It is also in the Kalama Sutta, AN 3.65:

"Then, Kālāmas, that noble disciple, who is thus devoid of longing, devoid of ill will, unconfused, clearly comprehending, ever mindful, dwells pervading one quarter with a mind imbued with loving-kindness … with a mind imbued with compassion … with a mind imbued with altruistic joy … with a mind imbued with equanimity, likewise the second quarter, the third quarter, and the fourth quarter. Thus above, below, across, and everywhere, and to all as to himself, he dwells pervading the entire world with a mind imbued with equanimity, vast, exalted, measureless, without enmity, without ill will."

http://suttacentral.net/en/an3.65

The way that I explain it to folks is:
Buddhist Virtues (The Four Immeasurables)

Compassion: The intention and capacity to relieve the suffering of oneself and all other living beings.
Loving Kindness:  The intention and capacity to bring joy and happiness to oneself and all other living beings.
Empathetic Joy: Rejoicing in the happiness and virtues of another living being
Equanimity: Accepting loss or gain, praise or blame, success or failure with detachment.  Regarding all living beings equally. Being clear-minded and tranquil but not dull.


text:
What determines whether actions are good or bad?
In a large number of religions, 'good' and 'bad' are defined by some all-powerful diety who either sends a representative or himself to tell us about what exactly they want us mortals to do.
In Buddhism, instead an enlightened individual basically worked it all out.
So the basic question is - why does that happen? What decided that killing (even if your intentions are good*1) will cause something negative ? What about the other actions? There is no 'final judge' to confirm or deny we've followed a particular path - so why does the universe (?) act this way? And what makes us sure that we're correct?
*1 - It's an impression I got from reading some other answers, if I'm wrong on this please do correct me)

Suggested reading: Good, Evil and Beyond Kamma in the Buddha's Teaching

In Buddhism, the concepts of good and evil do not exist as in other religions (no being is inherently good or evil). No deity tells you what to do. There are the teachings of the Buddha, that we can make use of, as opposed to believing in a doctrine (such as "Ithe authority tells you this, therefore you should obey*").
However, there is still a distinction between the helpful and harmful, skillful and unskillful - in the context of the path (to liberation, nirvāṇa, nibbāna). In terms of karma (actions, kamma), there are the wholesome and unwholesome, but these are useful in terms of the actions we choose to avoid and to act on, in each given moment (now).
Mara, as mentioned in the Pali Canon, is referred to as evil (in the English translation), but even he will be enlightened one day. In other words, noone is inherently evil (or good) - no deities, ghosts, humans or other, in Buddhism. For instance, Maha Moggallana, disciple of the Buddha, mentioned to Mara at one point that in a previous existence, he had been Mara, and (the current) Mara had been his nephew. Strikingly, despite having been the "embodiment of evil in the world", a number of lifetimes later, he was one of the prominent disciples of the Buddha (Gotama).
An excerpt from the story: (source)

The first Buddha appearing in our "fortunate aeon" (bhadda-kappa) with
  five Buddhas, was Kakusanda. He lived when the lifespan of man was
  40,000 years and when the first darkening of the golden age became
  evident because of a king's lack of concern and the occurrence of the
  first theft. Because of that, man's vital energy became reduced to
  half. At that time, Moggallana was Mara, chief of demons, lord of the
  lower worlds, and his name was Mara Dusi. He had a sister by name of
  Kali whose son was to become the Mara of our age. Hence Moggallana's
  own nephew was now standing in front of him at the door post. While
  being the Mara of that distant time, Moggallana had attacked a chief
  disciple of the previous Buddha by taking possession of a boy and
  making him throw a potsherd at the holy disciple's head so that blood
  was flowing. When the Buddha Kakusandha saw this, he said: "Verily,
  Mara knew no moderation here" — because even in satanic actions there
  might be moderation. Under the glance of the Perfect One the astral
  body of Mara Dusi dissolved on the spot and reappeared in the deepest
  hell. Just a moment ago he had been the overlord of all the hellish
  worlds and now he himself was one of hell's victims. A moment ago he
  had been the greatest torturer and now he himself was undergoing one
  of those terrible torments.

Whether we believe in the details of the stories in the Pali Canon is up to each of us. However, important to note, is that we all have the ability and opportunity to choose a wiser path.
Regarding your questions (in shuffled order):

There is no final "judge". No being (God or otherwise) controls our existence.
The Buddha (through long practice) awakened to (discovered, if you will) the way things are, how this existence functions.
Upon awakening, the first inclination of the Buddha was that "noone will ever understand this" - that it's impossible to teach. However, he was persuaded that some beings could be helped, and so he decided to teach others.
Briefly explained, words are inadequate to explain the way things function (karma, and so on), so instead of preaching something to believe in, the Buddha pointed the way for each of us to find out.

So noone "decided that killing is negative" (nor did anyone decide the effects of other actions). However, Buddhists will gladly help you on the path to finding out for yourself, to discover the effects of karma (your actions). Regarding being "sure we're correct", don't just take anyone's word for it. Try it out, see for yourself, do the methods and teachings from Buddhism work for you. Only then can you find certainty.

"why does that happen? What decides that killing (even if your intentions are good) will cause something negative ? What about the other actions? why does the universe act this way?"
As some answers correctly say, an action is bad when it leads to suffering, imprisonment, and confusion - and good when it leads to peace, freedom, and correct understanding. But why? What makes an action wholesome or unwholesome?
The nature of suffering, imprisonment, confusion - is conflict. A mismatch between how things are and how you want them to be. A conflict between you and the world. A conflict between people. An inner conflict in one's psyche.
The nature of peace, freedom, truth - is no conflict, no mismatch between expectations and reality, between us and them, no inner conflict, no outer conflict.
So, things that lead to conflict, to division, to mismatch - are bad. And what are these things? Lies lead to mismatch between beliefs and reality, therefore lies are bad. Divisive speech leads to conflict between people. Desire leads to unsatisfied expectations, which is a form of inner conflict. Desire also leads to conflict with people who have what you want. Hatred leads to conflict. Intoxication leads to mismatch between perception and reality. That mismatch is a form of conflict. And so on. Whatever action spreads the seeds that grow as division of people, division of mind, conflict - is going to be unwholesome.
The same principle works for wholesome, positive action. Whatever leads to unification, to decrease of conflict, to less division, to a better match between ideas and reality, between people, and within one's own mind - that leads to harmony, to peace.
Truth, peace, love, acceptance, suchness - is conflictless.
Non-truth, war, hatred, intolerance, dukkha - is conflict.
Conflict and conflictlessness is the key principle behind Buddhist ethics. This is what defines whether the action is good or bad, through the mechanism of cause and effect.
Divisive action leads to division. Conflict-inducing action leads to conflict. Un-truthful action leads to conflict of information. Violence leads to conflict.
Reconciling action leads to peace. Pacifying action leads to no-conflict.
This is the basic principle. Conflict is bad. No conflict is good.
P.S.
The nature of suffering is conflict because mind (being an informational phenomenon) does not handle logical contradictions well. So anything that leads to an experience of clash or contradiction is perceived as suffering by the mind attached to (or identifying with) one of the sides of the contradiction. Conversely, reducing the conflict leads to experience of harmony, and that's the mechanism behind 8NP. This is already down at the level of the fundamentals; I don't think there is a further why more fundamental than this.

If we define good as correct understanding, doesn't that make good relative to what someone perceives as correct. But, truth, reality, and expectation are different to different people. So, my action to me might be good as it accords with correct understanding but to another it might be bad. So a skillful action (even not doing anything) might then lead to suffering for someone else since their "reality" or expectation is different. Does this mean that conflict will always exist as long as there are different "realities"? An ultimate reality would then be a requirement to avoid this conflict?

Correct understanding means ultimately correct, i.e. Enlightenment, understanding of Buddha, seeing things as they really are. In Mahayana this implies direct realization of emptiness. In Yogacara terms, this is realization of Pariniṣpanna Svabhava.

Suffering/samsara is "bad". The end of suffering/nirvana is "good".
Therefore, whatever is conducive to enlightenment is "good", while whatever leads one away from it is "bad".

It is not always helpful to see Samsara as bad. We all experience Samsara (the repeating cycle of birth, life and death), it is the material we have to work with, so to speak. It just is what it is. Seeing it as bad opens for the risk of aversion to it - wanting to get away from it. However, craving/clinging and aversion are conducive to staying in Samsara.

Part of what makes an action good or bad is the intention.
Actions taken with unwholesome intentions such as these from "The Roots of Good and Evil" can be considered bad actions:

Greed — liking, wishing, longing, fondness, affection,
  attachment, lust, cupidity, craving, passion,
  self-indulgence, possessiveness, avarice; desire
  for the five sense objects; desire for wealth,
  offspring, fame, etc.
Hatred — dislike, disgust, revulsion, resentment, grudge,
  ill-humour, vexation, irritability, antagonism,
  aversion, anger, wrath, vengefulness.
Delusion — stupidity, dullness, confusion, ignorance of
  essentials (e.g. of the Four Noble Truths),
  prejudice, ideological dogmatism, fanaticism,
  wrong views, conceit.

Actions taken with wholesome intentions such as these, from the same book, can be considered good actions:

Non-greed — unselfishness, liberality, generosity; thoughts
  and actions of sacrifice and sharing; renunciation,
  dispassion.
Non-hatred — loving-kindness, compassion, sympathy,
  friendliness, forgiveness, forbearance.
Non-delusion — wisdom, insight, knowledge, understanding,
  intelligence, sagacity, discrimination,
  impartiality, equanimity

However there is also a factor of the end result. Some actions taken with good intentions can lead to suffering while some actions taken with bad intentions can turn out actually well for someone. So in another sense, actions which lead to suffering are bad actions and actions which lead to freedom from suffering are good actions.

Sensual pleasures aren't evil. It becomes evil only if the 3 roots of evil are present. Even Arahaths eat tasty food. It's said that they experience the taste best among all beings.

Yes, they are devoid of attachment, aversion and ignorance. So they do not cause evil or create new Karma

In my understanding, which is imperfect, in a Buddhist understanding of reality, there is no absolute moral ground to find or be found on, no inherent "way of life" to live up to or learn, no true measure of worth or worthiness, no true absolute bar or standard of what is to be considered inherently good or bad, no central value judgement system, and therefore nobody to communicate that system to us from some sort of divine moral source.
It is up to each individual to discover first hand what is beneficial, what is wholesome, what is skillful, what is good. We decide what that measure should be, and set it simply based off of experienced reality: good is what liberates us and alleviates the experience of suffering, bad is what confines and deludes us, that which increases the experience of suffering. Good and bad, therefore, can be inferentially equated to ignorance vs. awareness of what our true nature is really all about. This can be brought back to an awareness of the lack of inherent good or bad value.
So, good is that which is in line with this awareness of a lack of inherent goodness, and bad can be thought of in terms of ignorance, or the deluded notion that things have an absolute positive or negative value. This is tied into relativism, into emptiness via dependent origination, to interdependency, to what Thich Nhat Hahn calls interbeing. We have a good and bad, but it is merely a convention, in no way absolute. We can say that, as long as we have apparent existences, causing pain and suffering to others merely furthers our own misery, and misery in general, and ought to therefore be considered a "bad" or non-beneficial action.
If you know, and by "know" I mean like how you know that breathing is good and sleep is nice, what your true nature is, then you will understand on an intuitive level how to free yourself from and avoid ultimate suffering, and what is the absolute extinguishment of all suffering? :) This is the same as if you know that a fire burns from first hand experience, you call the act of sticking your hand in a fire bad, due to the effect that that will have: suffering.
This happens, clearly, because certain causes have certain effects. For instance, if you commit a horrible act to someone, you will eventually come to grips with the notion that this was a human, like you, vulnerable and suffering the vicissitudes of life. This will bring about the experience of suffering, and will cause you to either become numb or more sensitive. It must be contended with in some way. This is a form of suffering.
More so, you were suffering before you committed the act that led to further suffering. In other words, you had a feeling of suffering that acted as the motivation behind an act. That intention/motivation/feeling of suffering becomes stronger when you act on it, though we are seeking to satisfy ourselves. So it is understood that, ultimately, it is this intention that defines an act as having moral value, it is intention that creates good or bad karma, also perception, also effect. However, in Buddhism, the "aim" is to free yourself of karmic conditioned existence. It is seen that even "good" karmas are the causes of suffering, and are just a part of the truly good or truly bad trap.
So, are your actions set with the intention of extinguishing suffering? If so, with a correct understanding of what causes suffering, then they might be beyond karmic consequence. Are they set with the notion of true existence and value? If so, they will likely act as seeds, as cause for immediate or future suffering, and are ultimately non-beneficial towards the goal of ultimate, final liberation.
Question must be in the "why" of things, and have room for weather or not the apparent actor is aware of the consequences or effects of the actions that are being committed in the name of said intention. So, is there a killing where the intention was truly beyond the perception of absolute good or bad, motivated purely by compassion and free from all misapprehension and motivational suffering? I can't answer that for you, but I cannot imagine myself in a situation where a killing would not lead to more suffering. What decided that? I would argue just the bare, basic, un-elaborated, simple ole truth of our lives.
Remember, positive and negative are subject qualifications and are based on pleasure and pain, which are also subjective. We cannot value things in any absolute way given those truths. The universe does not judge you, you do a fine enough job doing that yourself.
There's a great story, and I will have to find the sutra this comes from, where a man came to the Buddha asking him to help, there was a dire situation. This man's brother (or some relative) had died, and they had not performed the necessary rights to ensure a safe passage to a heavenly afterlife, or good rebirth. He knew of the Buddha's fame as a sage and teacher, so thus sought out his expertise, certain that the great Buddha could alleviate this misfortune.
The Buddha instructed the man to go get two very light weight "jars", and fill one with melted churned butter, and the other with rocks, and bring them, with the body, to the river. Excited, the man did this, believing the Buddha would surely help his brother.
When he came back, the Buddha was waiting for him, and instructed him to place both jars in the river, and when he did so, they just floated there, slowly drifting away. The Buddha then broke open the first jar, and the butter stayed on top of the river as the jar broke. It frothed up, and was carried away. Then he broke the other jar, with the rocks, and the rocks all sank down after the jar was broken.
He told his new friend that there is nothing that anyone can do after the life is over. Only the weight of our good or bad deeds determine where we go in rebirth, as it does so from moment to moment in this life.
Our actions leave impressions in our consciousness, in our minds and on our hearts, based on our own sense of value judgements, which are based on our own sense of what is truly real and existent. These impressions carry us away, or sink us down. How is it not the case?
The trick is transcendence, to develop an awareness of the true nature of reality (anatta, anicca, dukkha) and get your intention fixed in a alignment with it. This is done by following the 8 fold path, and the morality section is based off of simply making it so that there are no mental distractions that will keep you from developing your awareness. As such, they are based off of non-attached, dispassionate, compassionate social activity. They are aimed at selflessness.
Karma is based off of volitional reactive activity to perception driven feelings, the quality of those feelings, and our relationship to them in terms of either our tolerance to them, or their influence over us. To reduce blind volitional reactivity, we can develop awareness and equanimity via meditation.

In Buddhist psychology, there is a very realistic answer to this question in terms of kusala sankhara and akusala sankhara. In essence, kusala sankhara consists of those causes of actions that are based upon awareness, knowledge, and compassion, while akusala sankhara consists of those causes of action that is somewhat lacking one or more of these qualities. The reality is that the causes of either form of behaviour are very complex, intelligent, and mostly unconscious.

Why does that happen?

Understanding the causes of human behaviour and human intelligence is a vast study. The Theravadin Abhidharma and modern psychology are examples of such studies. You ask the question as if there is a simple answer. Causality is complex and there is no simple way to characterize it.

What decided that killing (even if your intentions are good) will cause something negative?

Wrong question. The only knowledgeable question to ask is: How does an intelligent and well-informed person decide the psychological consequences of a particular instance of killing? The answer: He decides on the basis of a whole lot of background information. How can you expect otherwise? In psychology, there is no “what” that decides anything.

What about the other actions?

Same answer as #2, above.

There is no 'final judge' to confirm or deny we've followed a particular path - so why does the universe (?) act this way?

In psychology, there is no final judgement, only a very carefully qualified estimate. The universe does not perform actions. Only persons (and perhaps some animals) perform actions.

And what makes us sure that we're correct?

When it comes to psychological explanation, certainty is never an option. It sounds like you are talking about the attitude of a dogmatic guru. If so, I suggest you find another guru that does not have a deep psychological need to feel superior to you.

@ Ronald as you are talking from psychology perspective can I interpret your answer as, what your conscience tells you is good is 'good' and if it pricks its 'bad'.

No. Conscience deals only with what is obvious to the individual. When it comes to recognizing more subtle forms of the wholesome or the unwholesome, psychological insight through vipassana is absolutely required. When good judgment is needed, conscience often sees the wholesome as unwholesome or the unwholesome as wholesome. Buddhist psychology is a very advanced and sophisticated psychology. Knowing what is wholesome requires an understanding of the very sophisticated intelligence of the Bodhicitta and that is very very difficult.

'roots', also called hetu, are those conditions which through their presence determine the actual moral quality of a volitional act, and the consciousness and mental factors associated therewith, in other words, the quality of karma.
There are 6 such roots, 3 karmically wholesome and 3 unwholesome roots, viz.,: greed, hate, delusion (lobha, dosa, moha), and greedlessness, hatelessness, undeludedness (alobha, adosa, amoha).
Read more...

Actions are considered as having beginning, middle and end. And determinant is their end, sometimes called fruit (phala) or result (vipaka). I.e., wholesome (or good) actions in Buddhism are these that lead to pleasant or happy experience as their final result, and unwholesome (or bad) actions are these that lead to pain and suffering. Action that is unpleasant at the beginning still could lead to happiness and thus considered wholesome. Example is unpleasant meditation. Action that is pleasant at the beginning, easily could lead to the suffering as final result. To determine results of action need some sort of knowledge and wisdom (prajna). Otherwise action is conditioned by ignorance (vipaka-avidya) and commonly lead to suffering.
Good detailed article Kusala and Akusala as Criteria of Buddhist Ethics, Bhikkhu Thich Nhat-Tu.

Here's a non-traditional perspective:

All of our actions will have some effect on the world
Some effects are more desirable than others

Ethics are typically an extension of the perception of effects with regards to others as the self. As people are complicated and have greatly differing and conflicting desires, there is much to be considered when choosing the best course of action. Note that there are many compounding issues, such as the limitations of your own perspective, knowledge and intelligence.
Many buddhist teachings are to help guide one to typically good courses of action.
So, for your example, killing is almost universally considered non-optimal, because:

Most people enjoy living, and fear the unknown and rate death as an extremely negative experience
Most people enjoy other people, and do not want the other people around them to die

Therefore typically, death is an extremely negative outcome, not only for the person killed, and but for many of the people that person interacts with. In addition to that, there's also:

Death (discounting resuscitation) cannot be undone to any known degree.
You are probably wrong about something. Many things about a given situation are currently unknowable. It's impossible to truly know what a person was thinking, and, more importantly, what they will do in the future. Often we misattribute the reasons for a persons action based on the person themself, rather than their situation (there's a book about this). Because of this, its quite likely that our reasoning about another person is wrong. Much of Buddhism is about avoiding wrongs from this source of error (it's why intention is so important)

Because it's difficult to be certain, the action is undoable, and its a strong negative for many people, few do it. The only time it might be considered morally justified, if it was certain that greater harm would come from not killing the person; but again, this depends on the relative certainties involved, and this is were most disagreement about things such as the death penalty come from. This uncertainty in the certainty is also a meta-factor; the permanence of death will almost always spark some disagreement, which then becomes a weight in determining if it is a right action.

Intent.  Nothing less, nothing more.
An action is good simply if the intention was good.
And bad if the intention was bad.
Consequences and results don't change this.
If an intent was good, which led to something 'bad' occurring, that's just how the outcome was subjectively judged.  What is a 'bad' outcome?  That's just someone's interpretation of the situation.  Two people could interpret the same results differently. One good, the other bad.
Same goes for intention.  Two people could perform the exact same action and expect the same outcome, but one could have a good intention, and the other bad.  So the same action could be either good, or bad.
Why would Karma, for example, 'punish' a soul for an intentionally good action when the result turned 'bad'.  Where would the guidance be?  What would be the reason of Karma?  To punish bad luck?
So now that you know this, the only good response is an up-vote! :)

I think violation of five precepts constitute bad action and keeping of five precepts constitute good action.

The five precepts are: The precept that is abstaining from killing
  beings; the precept that is abstaining from taking that which is not
  given; the precept that is abstaining from sexual misconduct; the
  precept that is abstaining from false speech; the precept that is
  abstaining from intoxicating beers, wines and spirits causing
  heedlessness.

Answering your questions :

why does that happen?

It happens because of raga,dosa and moha.

What decided that killing (even if your intentions are good*1) will
  cause something negative?
  What about the other actions?

Nobody decided. It is merely an observation that killing causes bad karma. Similarly lying is bad karma.

There is no 'final judge' to confirm or deny we've followed a
  particular path - so why does the universe (?) act this way? And what
  makes us sure that we're correct?

Universe acts in this way because there is far greater suffering if we go down the wrong path. In order to remain on the right path ,the path leading to cessation of suffering,we must follow five precepts. We are sure we are correct because Buddha the great Teacher said so. You can test the validity of five precepts yourself but that would be a waste of time.

If you perform your action out of awareness, consciously aware of the present moment, then that action will be a good action if the same action will be percormed mechanically, without being aware it will be bad.
If you are conscious, you will be using all your intelligence to do the action...and then again its been repeatedly pointed out in many book I have been reading that 'we humans don't have free will'. Even Ajahn Brahm says that. So if I don't have real choice of my actions how can I be held accountable for and then the question of good and bad should become irrelevant.
So practise present moment awareness, what will follow will be good as it invarently be in accordance will kamma.


text:
Anger management - How to manage bad temper?
Is there any meditation techniques that we can used to manged Bad Temper. As I know it is possible to manage angers by observing it as "feeling angry.., feeling angry!".
However the problem is with the bad temper, sometime it is little bit too late to observer the anger. Any tips to improve the mindfulness to minimise the time gap?

I used to have a problem with anger.  I have found that mindfulness meditation (in my case, part Samatha and part Vispasana) works well but that the most powerful tool is compassionate meditation, also called Metta meditation.
Here's an example from http://viewonbuddhism.org/Meditations/equanimity_compassion_meditation.html:

Visualise in front of you three persons: at the left a good friend, in the middle a stranger, to the right an enemy or someone you cannot stand.
  - Concentrate on the friend in front and examine your feelings towards him or her.
  - Now concentrate on the stranger and examine your feelings towards him or her.
  - Now concentrate on the enemy and examine your feelings towards him or her.
  - Return to the stranger and realise that this person can easily become your friend or enemy in the future.
  - Next, look at the friend and realise that this person may become your enemy in the future when cheating or hurting you.
  - Now, look at the enemy and realise that this person may become your friend in the future when helping you.
  - Again look at your friend and try to strongly feel love and appreciation.
  - Now look at the stranger and try to hold this feeling towards this person.
  - Again look at your friend and try to strongly feel love and appreciation.
  - Now try to hold this feeling while looking at the enemy; is it really impossible to feel some love and compassion for this person?
  - Try to realise that all three, friend, stranger and enemy are completely equal in trying to become happy and trying to avoid suffering.

What I personally do is use a Loving Kindness Meditation (LKM):

May you be happy
  May you be healthy
  May you be safe
  May you live with ease

First I focus on a loved one.  Then a friend. Then someone I feel neutral about.  Then someone I dislike.
In the moment if someone is verbally attacking me or being otherwise disquieting I think to myself, "This person is suffering. They think that what they are doing now will ease their suffering. But it won't."  And then I do the LKM silently to myself.
And at all times when I am in danger of losing my equanimity I focus on my breath.
Finally, I forgive myself if I fail and get angry.  I am not enlightened, I am just another traveler on the path.

Loving kindness or Metta meditation is the one recommended to counter anger. What you are talking about is a Vipassana meditation. Both work! But you need to keep practicing. The more you practice, the quicker you can observe. For me personally, observing the Anatta nature of anger helps to dissipate it fast.
"sometime it is little bit too late to observer the anger" - Even here you can notice Anatta. You are late to notice it, because it came into your mind without your consent. And you can't make it go away just by wishing. It means anger is not a part of you. Because it doesn't behave according to your liking. So, if it has nothing to do with you, why react? Furthermore, anger isn't even it's own self. Because anger cannot arise on it's own. If you saw someone and got angry, that required a functional eye, light hitting the eye, presence of a mind at that moment. Anger couldn't have arisen if any of the 3 was missing.

Samatha calms the mind and its formations. This leads the practitioner to abandoning the five hindrances.
Just note that your mind has wandered as soon as you realize it has happened. With enough practice you will immediately recognize when the mind has wandered to one of the five hindrances. With even more practice the mind will stop wandering. Eventually a practitioner will develop Jhana, the culmination of samatha practice, where they can attain a tranquil state instantly.
It just takes practice and patience.

Kill it. Uproot it from your heart by persistent meditation practice. Do a lot of metta to cleanse your heart from any kind of negative thought.

Having killed what do you sleep in ease? Having killed what do you
not grieve? Of the slaying of what one thing does Gotama approve?
[The Buddha:] Having killed anger you sleep in ease. Having killed
anger you do not grieve. The noble ones praise the slaying of anger
— with its honeyed crest & poison root — for having killed it you
do not grieve.
-- Ghatva Sutta