text:
How does stream entry occur?
When a person becomes a stream enterer/Sotapanna, I understand they no longer have the first 3 fetters of belief in a self, doubts about the Buddha, his teachings or the noble sangha, or attachments to rites and rituals. But what's not clear to me is the mechanics of it. Does a person have a glimpse of Nibbana in a moment of true mindfulness and the three fetters drop away? Or does a person work at eradicating the first 3 fetters and if successful have a taste of Nibbana? Or can it happen either way?

This is a great question with great answers.

Does a person have a glimpse of Nibbana in a moment of true mindfulness and the three fetters drop away?

Yes, this is the correct explanation. The path of purification is as follows:

The meditator undertakes the practice of morality, specifically guarding the mind from unwholesomeness (sīlavisuddhi)
Guarding the mind, the meditator cultivates focus, gaining clarity of mind based on the objects of experience (cittavisuddhi)
Having a clear mind, the meditator cultivates an understanding of the nature of experience as composed of impersonal physical and mental constituents (diṭṭhivisuddhi)
Observing the physical and mental phenomena, the meditator cultivates an understanding of the causal interactions between the physical and mental phenomena (kaṅkhāvitaraṇavisuddhi)
Through the understanding of positive and negative causal relationships, the meditator cultivates an understanding of what is an what is not the path (maggāmaggañāṅadassanavisuddhi)
Through an understanding of the path, the meditator cultivates the right path (paṭipadāñāṅadassanavisuddhi)
Through cultivating the right path, one attains knowledge and vision of the noble path and fruition (ñāṅadassanavisuddhi)

Sotāpanna occurs upon attainment of the seventh stage. The right path (#6) is the gradual understanding that all formations are impermanent, suffering, and non-self. Once this realization becomes all-encompassing, the meditator attains an absolute certainty of one or another of the three characteristics and this leads to a release based either on knowledge of signlessness (based on impermanence - that there is no telling what will happen in advance), desirelessness (based on suffering - that there is no benefit to clinging to any formation), or emptiness (based on non-self - that all formations are void of self and there is no relationship of ownership or control in regards to all formations).
This release leads to an experience of cessation, where there is no arising of sense experience (including mental sense experience). This is the realization of nibbāna, and this is what leads to the eradication of the first three fetters.

Wrong view is eradicated because one can never believe that anything could be permanent, satisfying or controllable, having seen them all cease without remainder.

Attachment to wrong practice is eradicated because one can never be confused about the practice that leads to nibbāna after seeing nibbāna for oneself.

Doubt about the Buddha, Dhamma or Sangha can never arise, because one knows what the Buddha taught to be true with complete certainty, and the results that one attains having followed said teachings.

Here are some source quotes from the Visuddhimagga:

Then, while every sign and occurrence appears to him as an
impediment,when conformity knowledge’s repetition has ended,
change-of-lineage knowledge arises in him, which takes as its object
the signless, non- occurrence, non-formation, cessation,
Nibbāna,—which knowledge passes out of the lineage, the category, the
plane, of the ordinary man and enters the lineage, the category, the
plane, of the Noble Ones,—which, being the first adverting, the first
concern, the first reaction, to Nibbāna as object, fulfils the state
of a condition for the path in six ways, as proximity, [673]
contiguity, repetition, decisive-support, absence, and disappearance
conditions,—which is the culminating peak of insight,—which is
irrevocable

and

The abandoning of the states beginning with the fetters by the
noblepath knowledge in such a way that they never occur again, like a
tree struck by a thunderbolt, is called abandoning by cutting off.
With reference to this it is said: “Abandoning by cutting off comes
about in one who develops the supramundane path that leads to the
destruction [of defilements]” (Paṭis I 27).
(Both from Nyanamoli's translation of Vism XXII)

Are there also relevant quotes from the suttas (rather than the commentaries)?

Stream-entry is very simple, there is nothing supernatural about it. Stream-entry is when the practitioner has fully understood the vision behind Buddha's teaching, has fully assembled the puzzle, and now has no doubts about what Buddha meant when he said certain things or why he said them. Having a clear vision of what Buddha was referring to includes clearly seeing the reality behind Three Marks of Existence and the actual mechanism behind the Four Noble Truths.
Because most of us are so dominated by our preconceptions about the world, and about Buddha-Dharma, stream-entry is difficult or virtually impossible to attain from mere theoretical study of sutras, or from one super-successful meditation session, when one gets a "glimpse of Nirvana". Instead, stream-entry requires an effort consistently applied over a long period of time, targeted at putting Sat-Dharma to practice in day-to-day life.
The reason it's called "stream"-entry, is not because one enters some magical stream, but because one finally sets foot on the unquestionably right road towards Nirvana, having really understood the whole thing.
As I understand, what he subsequently called stream-entry (attaining a clear understanding of Three Marks of Existence and Four Noble Truths -- and therefore cutting all doubts) was attained by Buddha at time of his Bodhi, but because he had already liberated his mind from most of the other asavas by then, he effectively achieved full Nirvana at once. But in case of Buddha's students (and ourselves), because we do have the advantage of teaching available to us, we can assemble the puzzle while still having residual mental/emotional obscurations. Hence the need to have a separate name for this stage, distinct from "bodhi".
As for how exactly stream-entry occurs. It occurs through exhausting samsara. In order to exhaust samsara you must exhaust ego. In order to exhaust ego you must explore all corners where Enlightenment might hide, until you know you couldn't have possibly missed it. Then it finds you :)

Thanks. Very well explained. Is it possible to fall back due to discontinuing meditation? Anagami to sagdagami. Sagdagami to sotpanna and so on.

For both logical as well as didactic reasons I have to say, yes, it is possible to regress. Not necessarily due to discontinuing formal sitting meditation (because there are advanced practitioners whose awareness grew so much that they effectively meditate-in-action) but mostly because of careless misdirected attention, which can lead to experiences which (when applied on top of residual ignorance) can gradually accumulate as new unhealthy samskaras. Can't fall back further than srotapanna though, that's for sure.

@Andrei Volkov Friend, I will quote the following from you: "For both logical as well as didactic reasons I have to say, yes, it is possible to regress." Now, are you trying to say that a person can regress from one stage of enlightenment to another (e.g: Anagami back to sakadagami)? If that's the case, can you do me the great favor of providing some sources? Amplifying what you meant by that would also be appreciated. kevin. Note: I don't have a 50 reputation to comment to what you said.

@kevin, during the 2nd council held 100 years after Buddha's Parinirvana, Sthavira (so-called elders, of which Theravada is a remote offshoot) separated from Maha-sangha (majority, which eventually developed into Mahayana). Most of the latter believed that stages up to anagami were fallible (due to residual ignorance), and some even held that arahants were fallible too (google "five points of Mahadeva"). Unfortunately most of the information about early extinct schools exist only in form of references in later texts. If you google for "arhats fallible" you should find plenty of citations.

What is made of the mind can be unmade by the mind. "Happiness and enlightenment are living things and they can grow. It is possible to feed them every day. If you don’t feed your enlightenment, your enlightenment will die. If you don’t feed your happiness, your happiness will die. If you don’t feed your love, your love will die. If you continue to feed your anger, your hatred, your fear, they will grow. The Buddha said that nothing can survive without food. That applies to enlightenment, to happiness, to sorrow, to suffering." http://www.lionsroar.com/in-engaged-buddhism-peace-begins-with-you

Eating healthy, exercising regularly, sleeping well, and so on I remain fit and healthy. Sometimes my body can compensate for minor lapses, other times it is less able. Enlightenment is a living thing like my body, though relatively unaffected by old age and sickness, if abused, it also regresses. Sometimes it has the ability to bounce back from abuse, other times it needs lots of care and attention to heal back to good health. Nirvana isn't a goal to attain and rest easy, even though the first moment of attainment is special, it is a state of mind that is continuously attained.

For people who didn't hear/know about Budha's teachings?

For stream entry to happen you have to go beyond the realm of mind and matter (Nirvana). Momentarily your faculties stop working you enter into Nirodha Samapatti / Phala Samapthi. When going in to this stake you see the links of Dependent Origination in the forward direction and when you come out of it you see it in the reverse direction. Depending on the intensity of your experience you enter into some stage of sainthood. If it the weakest it is stream entry.
Before you enter into Niroda you have to extinguish all Sankara leading you to births in the lower realm through Vipassana. Once you experience Nirvana your mind is at the sate that you cannot do any Karma giving birth in the lower realms (falling away of the 3 fetters) and you have eliminated all your store of Sankara which leads to birth in the lower reams hence you cannot be born in any of the lower realms.
(This is as per interpretation in the Ledi Sayadaw linage.)

Why the downvote? What is wrong or what needs improvement?

First paragraph:  That is exactly how the Bhante Vimalaramsi teaches it.  Plus, of course, there is the two-fold attaining: of the path and then, after that, of the fruit knowledge.

First parapgraph:  Is there an English-speaking source available, where the Venerable Ñanadhaja explains this?

I believe there are probably a few ways to enter the stream.
I have entered the stream (as a lay person)
Having no parents has really helped me to connect more deeply with the
true nature of the way things are. I have had little conditioning, no
preconceptions. As an autodidact, things have been revealed to me
intuitively over many years.
For the last 20 years I have been going on walks (walking meditation)
in the local forests asking questions. Nature has been my mother
and father, my teacher. My eye has always been open on these walks.
I realized that whatever arises would cease and it was heartwarming.
For me, being a stream winner is like this: When you have to overcome
extreme trauma and appalling loss and you don't become cruel and
you can forgive and you can let it go, you stop the cycle.
You have done what you came here for and you transform.
That and  maybe 20 years of walking in woods...lol. I doubt I could
do another 20! Be Well.

Factors for Stream Entry.

Then the Venerable Sāriputta approached the Blessed One, paid homage
to him, and sat down to one side. The Blessed One then said to him:
“Sāriputta, this is said: ‘A factor for stream entry, a factor for
stream-entry.’ What now, Sāriputta, is a factor for stream-entry?” (1)
‘Association with superior persons, venerable sir, is a factor for
stream-entry. (2) Hearing the true Dhamma is a factor for
stream-entry. (3) Careful attention is a factor for stream entry. (4)
Practice in accordance with the Dhamma is a factor for stream-entry.’
“Good, good, Sāriputta! Association with superior persons, Sāriputta,
is a factor for stream-entry. Hearing the true Dhamma is a factor for
stream-entry. Careful attention is a factor for stream-entry. Practice
in accordance with the Dhamma is a factor for stream entry.” - SN 55.5

Here is a book free for downloading (no breaking 2nd precept here :-)
http://watnapahpong.com/static_media/Sotapanna_Handbook_English_version_30.pdf
Buddha gave 50+ definitions (40+ in this book) of what stream enterer is and what not.  Also how to get there.   one thing certain, buddha allowed only him and yourself to predict if you have reached the state.
and as for Nirvana, nibbana.  it is not a place where 5 skandha is ..
vijñāna (consciousness) since it is not permanent (dukkha) ,thus, cannot take root or manifest  in Nirvana.  the 3rd of 4 noble truths does not describe nirvana but cessation of desires.   That is something  vijñāna (conscious) can take place.

With great respect, if stream entry is sought for, I can recommend 'Liberation Unleashed'.
This is a group run entirely free and by volunteers (of which I am one) who have also realised the emptiness of self, and are willing to guide others to the same realisation. Simply request a guide in the forum - Alternatively, visit us on Facebook and request a guide there.
http://www.liberationunleashed.com

Welcome! I hope you might add something to answer the specific question-as-asked which was something like, "Is stream entry sudden, or is it gradual?" or maybe it was about what is cause versus what is effect: "Do the fetters drop because of glimpse, or is the glimpse because of fetters dropping?" Perhaps you can edit your answer, either to include some extra information directly, or to identify any specific page/text within the web site you referenced, which might help to answer this question.

Hello and welcome to Buddhism.SE! We've put together some information to help you here started here.

This answer reads like an advertisement.

@ruben2020  Well technically it is an advertisement, but the product is great, and absolutely free. I have worked with two guides from LU and both were extremely patient, helpful and knowledgeable.

Does a person have a glimpse of Nibbana in a moment of true mindfulness and the three fetters drop away?

Yes. In a moment of true mindfulness and equanimity, nibbana can happen and when it does, the fetters drop away. That is why attainment of path is referred to as "clear knowing and release" in the suttas.

Or does a person work at eradicating the first 3 fetters and if successful have a taste of Nibbana? Or can it happen either way?

No, Nibbana happens first and that results in eradication of fetters. The behavioral changes might take a while to come into effect though, after the review cycles. Post stream entry, mind starts cycling through various insight stages, which result in changes in external behaviour.

First of all one becomes faith-follower by gaining conviction in regards the nature of these phenomena;

feelings
forms
perception
knowing
intentions

as;

not belonging to a self
not constituting a self
not being apart from a self
not being in a self

This is called safeguarding the truth having placed conviction, one does not yet come to a definite conclusion.
Further having pondered the nature of these phenomena as such one comes to a point where the nature of these phenomena has been penetrated by modicum discernment.
This stage is called Dhamma-Follower, the second type of a Noble Disciple, also he safeguards the truth.
As i understand it, in the Theravada Tradition the two type of Followers are thus classified as people working towards the realization of the first fruition and would be included in the 1st category in the 8 fold delineation of the 4 pairs of Noble Disciples, namely;

One working towards realization of the 1st path fruition
... of the 2nd path fruition
... of the 3nd path fruition
... of the 4th path fruition
One who has realized the 1st path fruition
...the 2nd path fruition
...the 3rd path fruition
...the 4th path fruition

The followers are however not included in the 5-fold classification of Noble Disciples, namely;

One attained to view
One liberated by faith
One liberated by wisdom
One liberated in both ways
A bodily witness

Therefore i assume that the list of 5 refers to types who have transcended the stage of Safeguarding the Truth because the one attained to view has seen with discernment and cut the three fetters and the other types are Arahants.
Therefore to transcend the stage of safeguarding the truth one needs to go beyond pondering and "see with discernment" as to come to know and see that the phenomena (feelings etc) are actually that way and only that is true and everything else is false;

the Dhamma being taught is deep, hard to see, hard to realize, tranquil,
refined, beyond the scope of conjecture, subtle, to-be-experienced by
the wise.
Exerting himself, he both realizes the ultimate meaning of the truth
with his body and sees by penetrating it with discernment.
"To this extent, there is an awakening to the truth.

This implies a direct experience of discerning the escape from the made;

There is, monks, an unborn — unbecome — unmade — unfabricated. If there were not that unborn — unbecome — unmade — unfabricated, there would not be the case that escape from the born — become — made — fabricated would be discerned. But precisely because there is an unborn — unbecome — unmade — unfabricated, escape from the born — become — made — fabricated is discerned. - Ud 8.3

After this "experience" one is a stream-enterer and has transcended the stage of safeguarding the truth and is one awakened to the truth.
What remains is further development of those same qualities for the final attainment of the truth and becoming one of the Arahants in the 5-fold classification of Noble Disciples.

Once one attain the enlightenment as stream entry, all four Noble Truths perceived at the same time eradicating the first three fetters. As a bhikkhu(not necessarily a monk), afraid of Sansara, working hard on meditation achieving sixteen stages of wisdom , come to reach the Phala and clearly know what is Nibbana  with Magga nana getting rid of all doubts on Buddha teachings. Out of sixteen stages of wisdom, even at fourth stage one can have clear view  of non-self, no wonder stream-entry have no single grain of the three fetters left. But how this detail is so subtle that if you wish to know , as one of Dhamma Characteristics, ready to take on, come and try to achieve it.

One of the strongest delusions we live with is the Self. We can't even begin to imagine being without it, and we even imagine it's somehow eternal and or immortal. But like everything else, it's impermanent, it's not a thing in and of itself (it's made up entirely of non-self elements), trusting in this delusion causes suffering, and breaking the delusion brings freedom. This is why we practice the way we do. When we experience seclusion, quiet, and calm, the obscuring and clouding effects of thoughts and experiences settle down. We return to who we essentially are, impermanent and self-less. Stream Entry is basically just returning from meditation to the every-day world and not losing the direct experience of that. Seeing the world in this way is why we become convinced. The mechanism is practical and makes plenty of sense. Of course this is an important first step, as old habits and lifestyles are still going to be in the way. Really not having any inclination towards the "delights" of samsara is much better.

"Sariputta, 'The stream, the stream': thus it is said. And what,
Sariputta, is the stream?"
"This noble eightfold path, lord, is the stream: right view, right
resolve, right speech, right action, right livelihood, right effort,
right mindfulness, right concentration."
"Very good, Sariputta! Very good! This noble eightfold path — right
view, right resolve, right speech, right action, right livelihood,
right effort, right mindfulness, right concentration — is the stream."
— SN 55.5

I experienced what I would call stream entry without knowing anything about Buddhism other than a typical, ignorant, western perspective.
I was meditating and in my minds eye I saw a radiant white pearl with smaller discolored pearls floating around it (in many ways like a solar system). As I began to investigate the discolored pearls, I realized they were my wants and desires. I realized, "these are not me"; they began to fall away until all that was left was the white pearl. In that moment I realized that the "white pearl" is within us all.
Next I experienced a feeling of death overwhelming me. I knew I was dying; it was the most fear filled experience I have ever had. Then I felt reborn. In that moment I was only a body and a stream of consciousness. After that, all that was left was a feeling of love.
I can not say from experience but I believe that the same insight could come from persistent determination to remove the three fetters.
To clarify, this can be seen as reverting the 12 link chain, experiencing cessation and then being reborn

i'm no expert, but wouldn't the fact that this experience involved specific perceptions and feelings—make the experience not a glimpse of nibbana, and therefore stream entry did not occur?

@qweilun These are the moments immediately proceeding stream entry and the effects after stream entry. The moment between death and rebirth was nirvana.

For me it was an instant. A moment of realization: that I "am" Nothing. As I looked at each layer that I thought was "me" I recognized that "this is not me". After peeling back every layer I realized there was nothing left, yet I was still here. How could that be? THat must mean I am "this". And it was in that instant that I realized my true nature. It didn't have any "mechanics" to it. It was just a realization. The only mechanics, if you could call it that was an intense desire to know what I am and doing everything possible to find that.


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Health Risks of Meditation?
One hears a lot about the benefits of meditation; everything from calming yourself and lowering your blood pressure to working towards purifying your mind and finding freedom from suffering. Even as a newer meditator, I feel a better sense of physical and emotional well being.
But I'm interested to know if experienced meditators and teachers know of a down side to meditation either physically or mentally. Does so much time spent in one position lead to physical problems such as circulation problems or back issues or even mental breakdown during intense periods of meditation? And if so, are there things one can do to avoid such issues that might interfere with continued meditation?

I heard a monk once say that there are no health risks with (implied their kind of) meditation, but you risk losing your hair from prolonged use. :-)
NOTE: ^ meaning you'd become a monastic in case that was unclear.

Here is a physical problem: Knee injury from sitting postures without requisite hip flexibility. This applies to almost any floor-sitting posture, depending on the meditator's hip flexibility. This is based on my own experience injuring my knees in sitting meditation.
The knee injury occurs when insufficient hip flexibility causes the knee joint to rotate laterally. Lateral, in anatomical terms, means away from the body in a left/right direction.

The knee is a very simple joint mechanically, and is only designed to flex and extend. It is damaged by rotating medially/laterally. The lateral rotation required for sitting postures should come from the hip.

Full lotus position (each foot on top of opposite thigh) and half lotus position (one foot on top of opposite thigh) are the most dangerous positions for this case since they cause a lot of lateral rotation in the knees, if there is not sufficient hip flexibility. Burmese position (one foot in front of opposite shin) is much less dangerous, but might still injure a person with tight hip ligaments.
So the key is that we need flexible hips if we want to sit on the floor easily.
Although full lotus and half lotus position are discussed more in Yoga, and not in the context of Buddhist meditation, I feel this advice still applies since I injured my knees by sitting in half lotus. Now I protect my knees by avoiding lateral rotation, keeping my hips flexible, and only using Burmese position for sitting.
This video by KinoYoga shows how to prepare for lotus position safely, and can help to clarify by providing visual instruction.
http://www.youtube.com/watch?v=BdVHk5C0L8U
And this page has many stretches I have found useful to loosen the hip for safer sitting:
http://zenmontpellier.voila.net/eng/lotus/lotuseng.html
Be well and take care of your knees.

There are two types of meditations - calm  abiding (Tib.: Shine, Skr.: Shamata) and insight meditation (Tib.: Lhaktong, Skr.: Vipassana).
The meditation of calm abiding directs mind towards one location to calm it and techniques involve focusing on an object (a stone or a Buddha form) or a formless breath. Practitioners learn not to follow one's thoughts and after some training the mind can become undisturbed like a surface of a lake.
Once the mind is calm, one can practice insight meditation which is directed at recognizing the nature of one's mind. In Vajrayana tradition insight meditations involve visualising vivid energy forms of Buddhas, receiving lights from them and melting with them.
Calm abiding meditation is suited for everyone. The insight one, however, should not be practised by people who suffer from mental diseases and take psychoactive drugs. As said earlier, to practice insight meditation one should already have a bit of stable mind. One can compare it to the physical workout. If you got injured, first focus on bringing your body back to its stable state before embarking on a strenuous preparations for a marathon. If you disregard your injury, the marathon preparations will damage your body further and even seemingly simple actions like walking will become difficult.
Although there is no hard rule, by psychoactive drugs I mean prescribed medications used in psychiatric treatments. I've heard of a person with some anxiety disorder who tried to visualise a vivid Buddha form and receive lights from it. It was too intense and fearful experience for him as there was too much sudden stimulation. Clearly it was harmful for him and probably now he has rather negative connotations about Buddha. Recreational use of some psychoactive substances is also usually frowned upon. Insight meditation is a profound practise and best if learned from a teacher or some experienced practitioner. One should always have a chance to discuss the details and openly ask the teacher whether a given meditation is suited to his/her condition. On the internet or books one can find general guidelines, but the most important one - individual advice, is always received in person.
As for the physical health, the practice of prostrations can cause some damage if done improperly or in too many numbers per day. Again, if one already has knee of back problems, it is probably better not to them at all.

Caffeine is a psychoactive and I do not understand how that would hinder insight to such a degree that insight practice should be avoided. Can you elaborate on what you mean by psychoactive drug?

@user70 I'm no expert! Please do not read what I say as being from someone with lots of experience or expertise, but that said I could see how even a mild stimulant like caffeine could get in the way of the mindset you need to be in -- especially if you are sensitive to its effects. It's not just that it could "hinder" your progress, as much as it could take you down the wrong path altogether and lead somewhere potentially damaging. Meditation is a very powerful thing and should not be treated lightly.

I believe that even Shamata meditation can be counterproductive in certain phases of mental illness (i am not any kind of health professional though)
For instance meditation is now regarded as beneficial for depression. However in the acute phases of depression it might not be as effective or even counterproductive. To quote from the Wildmind website

As one experienced meditator said, “Meditation while clinically
  depressed can result in intensification of feelings of despondency,
  hopelessness, and negativity generally. The metta practice is
  theoretically a good thing, but in practice it can be a nightmare if
  all you feel is self-hatred!"

I would always be tempted to learn under a teacher rather than be tempted to go it alone. Any kind of specific health issues (mental or otherwise) can be discussed with them. Just to be on the cautious side.

I have heard that sitting in full lotus can harm areas of the legs. Also that sitting too long can cause blood clots. This is avoidable; walking meditation and even meditating during menial tasks (e.g., washing dishes, showering, painting, etc.) are practical approaches. Eventually a practitioner can meditate during any and all activities.
In a certain sense, mental breakdown, is a beneficial result. I have never heard of a negative mental breakdown from meditation.

One doesn't have to be in the full lotus position during seated meditation, so if that position causes harm, other positions (e.g. Burmese) can be used.  In my experience, a minority of people meditate in the lotus position when sitting.  This webpage illustrates several sitting positions.

Dr Willoughby Britton, a research and clinical psychologist, is doing a lot of work at Brown University on the so-called "dark side" of meditation. More information is here: http://cheetahhouse.org
There are also some videos (same site) of her interviewing Daniel Ingram talking about the same thing, and specifically that subset of the stages of insight that Ingram named "The Dark Night" (alluding to experiences very similar in appearance reported by some Christian mystics -- e.g. John of the Cross, and Teresa of Avila).
Britton's emphasis is not so much on anything inherently health-risky in meditation; overall she is positive, and I think is a practitioner. Rather she works on what appears to be not uncommonly encountered difficulties associated with making progress in insight. Ingram -- also a medic, albeit not a specialist like Britton -- is the same, although Daniel draws too much attention to it for my liking. The aim of both is to help people deal with the tougher side of things.
There's also a video by Shinzen Young on Youtube where he mentions this same idea. I can't find it, but I mention it as worthwhile because Shinzen gives a good balance to Daniel. Daniel scares the bejeezus out of you (on purpose, although, as I say not ideally) but Shinzen downplays it (to, I think/hope, the right level), saying that although difficulties can be encountered, they're not common, are rarely that severe, and are typically easily worked through provided you have a good teacher who knows what they're seeing -- i.e. usually the dukkha-nanas -- when they see them.

Just watched, on Britton's Vimeo site, a very interesting video conversation between her and the Dalai Lama where she outlines her findings on the negative aspects of meditation. What's striking, for me, is the Dalai Lama's reaction. I thought he'd have been all "Oh sure, I've seen that happen a lot". But in fact he's partly puzzled, partly troubled, and in general is clearly not of the view that such negative stuff is to be expected. His "diagnosis" is that such things happen if you try to meditate without a broader supporting context (especially ethical).

Here is the Shinzen video I walked about: https://www.youtube.com/watch?v=9zIKQCwDXsA . I thought twice before including it because I really don't want to add yet more hype to this area, since I think it, like terrorism and pretty much every other threat at least in the TV-soaked modern USA, is over-hyped. However, it's useful material and even though Shinzen talking about tough stuff, listen carefully to his own caveat starting  at 4:41.

Another interesting video on this stuff is https://www.youtube.com/watch?v=9VjU_505i6E. It's an Anglican vicar spending some time in solitude in the Egyptian desert, a la "The Desert Fathers". The connection to this thread is in the comment that Father Lazarus makes to the guy just before he starts his alone period. He mentions that if he can get through the first week or two, then he'll be fine. The implication is that with contemplative Christianity, just as with some Buddhism, a "dark night" is just part of the deal.

This excerpt from Q&A about Goenka-style Vipassana meditation should provide some answers to your question:

Is there anyone who should not participate in a course?
Obviously someone who is physically too weak to follow the schedule will not be able to benefit from a course. The same is true of someone suffering from psychiatric problems, or someone undergoing emotional upheaval. Through a process of questions and answers, we will be able to help you decide clearly beforehand whether you are in a position to benefit fully from a course. In some cases applicants are asked to get approval from their doctor before they can be accepted.
Can Vipassana cure physical or mental diseases?
Many diseases are caused by our inner agitation. If the agitation is removed, the disease may be alleviated or disappear. But learning Vipassana with the aim of curing a disease is a mistake that never works. People who try to do this waste their time because they are focusing on the wrong goal. They may even harm themselves. They will neither understand the meditation properly nor succeed in getting rid of the disease.
How about depression? Does Vipassana cure that?
Again, the purpose of Vipassana is not to cure diseases. Someone who really practices Vipassana learns to be happy and balanced in all circumstances. But a person with a history of severe depression may not be able to apply the technique properly and may not get the desired results. The best thing for such a person is to work with a health professional. Vipassana teachers are meditation experts, not psychotherapists.

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What are the texts that contain words which can be attributed directly to the Buddha?
If I understand correctly, the Buddha never wrote anything but some Buddhist texts quote him by describing meetings or situations where he was present. Is there a comprehensive list of all scripts where he is directly and textually quoted?

The Pāli Canon (Pali: Tipitaka) is the standard collection of scriptures in the Theravada Buddhist tradition, as preserved in the Pāli language. It is the most complete extant early Buddhist canon. It was composed in North India, and preserved orally until it was committed to writing during the Fourth Buddhist Council in Sri Lanka in 29 BCE, approximately four hundred and fifty four years after the death of Gautama Buddha.

See Pāli Canon on Wikipedia, also called the Tripiṭaka or Tipitaka, meaning "Three Baskets." I would recommend these books as a good reference to access original Buddhism.
Here is an example from the Mulapariyaya Sutta, Majjima Nikaya section of the Pali Canon.

I have heard that on one occasion the Blessed One was staying at Ukkattha, in the shade of a royal Sal tree in the Very Blessed Grove. There he addressed the monks, "Monks!"
"Yes, lord," the monks responded.
The Blessed One said, "Monks, I will teach you the sequence of the root of all phenomena [or: the root sequence of all phenomena]. Listen & pay close attention. I will speak."
"As you say, sir," they responded.

This is an example of how each "Sutta" starts.

Within Buddhism there is the notion of Buddhavacana which is the word of the Buddha i.e. those world traditionally attributed to the Buddha. A lot of the Pali Canon does start with an attributation directly from the Buddha e.g. the Sigalovada Sutta sutra begins

This is what I heard.
On one occasion, the Buddha1 was living near the town of Rajagaha2
  at a spot in the Bamboo Grove called the Squirrel's Feeding Place.

But some other parts of the Pali canon doesn't read like that. The Dhammapada which is a good candidate for the original word of the Buddha in a historic sense doesn't read like that at all e.g. from the Twin Verses section at the beginning (brilliant incidentally)

Mind precedes all mental states. Mind is their chief; they are all
  mind-wrought. If with an impure mind a person speaks or acts suffering
  follows him like the wheel that follows the foot of the ox.

And other later words do start with a direct attribution to the Buddha but one would need to be very willing to accept the supernatural aspect of Buddhism if you were to believe that these represent actual historical events e.g. the White Lotus Sutra begins

This is what I heard. At one time the Buddha was in.....

So a lot of texts are attributed to the Buddha in a religious sense (the White Lotus sutra) but aren't actually good candidates for the historic word of the Buddha. Historically the Pali Canon is the best bet for that (like senshin says) but the word of the Buddha can be given a looser interpretation - or if you like a more devotional aspect.

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How should Buddhists handle prayer requests?
Fairly regularly people ask me to pray for them or their family members. In my understanding, Buddhist don't generally pray for things to be different than what they are. But it never seems appropriate to mention this to a person who might be upset so I just say "of course" and leave it at that.
But the truth is, outside of metta for everyone, I don't pray for such things as my neighbor's uncle's knee replacement operation or for my co-worker's daughter to advance in the state championship for soccer. So I'm lying to someone and that's uncomfortable and not a good mind state to be in.
First, is my understanding that Buddhist don't generally pray for things to be other than what they are wrong view? And if not, what might be a better way to handle prayer requests?

This is a great question, certainly understandable. I don't think it's right to say that prayer as an activity goes against the Buddha's teaching; certainly prayer that stems from desire is unwholesome, but prayer that stems from kindness and purity of mind, in the sense of being an expression of one's attitude towards all beings, as:

Sukhino vā khemino hontu sabbe sattā bhavantu sukhitattā "May all beings be happy and secure, with a heart full of happiness!"
(Karaniya Metta Sutta)

is proper, since it leads to tranquility of mind and an outlook based on purity of intention. The problem is only when prayer is based on a sense of need for things to be other than what they are, rather than an outlook that informs one's life (i.e. acting, speaking and thinking for benefit rather than harm)
Another "wrong" regarding prayer, according to Buddhism, would be the localization of the prayer as depending on an external entity (i.e. God). Obviously if by prayer one means the beseeching of an external entity to intervene, that is likely based on both desire and wrong view.
So, in brief, when someone asks you to pray for them or someone else, by all means, say you will if you want; just be clear that to a Buddhist, prayer is simply an expression of "May so and so be happy and well." That's all anyone wants, is for people to be thinking kindly of them, and it is certainly a means of cultivating harmony and peace, both internally and externally. Just don't succumb to desire for change or expectation of outside intervention.

In Mahayana Buddhism one takes a Bodhisattva Vow which in essence means that one practises in order to liberate all the sentient beings as their well-being is more important than one's own. Thus, if asked for help, those who took the vow will try to offer some sort of help.
I took the vow and renew it at least once a year. When people ask me for prayers, I reply that I will make lots of good wishes for them and that I will keep them in my heart while meditating or repeating mantras. I never make detailed wishes though. Most people, including myself, have no idea what is good to them so I just wish that the best possible will happen to people so they can reach lasting happiness. Sometimes the best for their development is passing an exam, sometimes it is actually failing it.
In that way, I do not have to lie, I do make wishes for individual people (along with my everyday wishes that all the sentient beings reach enlightenment) and people feel a relief that someone cares about them. And after some time one of my friend stopped using a word 'prayer' and now asks me for good wishes:)

Apart from doing Metta, Buddhists have a practice called 'Saccakiriya'. It basically uses the power of true speech. But it has to be something strong.
ex:
I have never lied in my life! If that is true, may XYZ happen!
I have never stolen in my life! If that is true, may XYZ happen!
I have never killed in my life! If that is true, may XYZ happen!
Once the Buddha instructed Venerable Angulimala to invoke his Saccakiriya to help a pregnant lady.
This is what he said: "Since I was born of Aryan birth, O sister, I am not aware of having intentionally deprived any living being of his life. By this asseveration of Truth may you be well! May thy unborn child be well!"
You can also chant the virtues of the Buddha, Dhamma and Sangha and wish someone well.
Or you can venerate the Bodhi tree and wish someone well.

Sankha, thank you for adding this, but I'm not sure I understand it. This is only for people who have told the truth always, never taken anything without permission or killed even a bug? I think it might be too late for me! That is one of the issues of having grown up with the Ten Commandments instead of the Five Precepts. In the Ten Commandments, "thou shall not kill" refers to human beings. So I don't think I qualify to practice Saccakiriya. How does one venerate the Bodhi tree?

Yes, the power of the Saccakiriya depends on the virtuousness associated with it. Saying that "I ate rice at least once a day" doesn't fall under Saccakiriya even though it may be true.
But you can say "since the day I took refuge in the Buddha, Dhamma and the Sangha, I have never killed/stolen/lied...". You can also do good deeds like giving alms on behalf of your loved one and wish him/her well.
Here's some info on Bodhi Puja: http://www.bps.lk/olib/wh/wh402-p.html#2TheBodhiPj

Let's say there are two ways of dealing with this:

Your friend says: "Please, pray for me... I hope everything will be OK.".
You say: "Such a poor person... I hope and I pray that he will make it through and ...".

It has nothing to do with the physical and ceremonial praying part.
It stems from Compassion and Kindness.
Regardless of which cases as mentioned above, your Compassion would reflect your action, not just towards the person who, before your eyes, is in need, poor, and in unfortunate situation, who you "directly" help (pray for).
It doesn't mean you would heal him or solve his problem at an instance (i.e. you become a doctor and heal him).
Instead, it goes further than that:
Your Compassion would be reflected in every little action that you have towards those in need, even if they may be trivial - it may be as simple as a smile; sharing your meal and bringing it to the neighbor, whose immobility affects his ability to cook / buy food; taking a little time even after a long day at work to speak with someone who is sick but has no-one to talk to but you take a little time even after a long day at work to speak with him; etc etc.
Along the journey, one might learn something from it as well and acquire more Compassion; a funny example:

You decide to go do some errand for this poor neighbor of yours, who is immobile due to sickness. At this supermarket, you happen to see some people standing outside of it, talking loudly and bothering passersby with their words. You are infuriated, and as strong as you are you could physically harm these "bad" guys with your might and teach them a lesson. But you didn't. Why? If it suddenly occurs to you that harming these people would put them in a situation just like your poor neighbor, your Compassion channels through, namely more than just caring for the neighbor - your Compassion becomes vast and endless and unbiased.

Through these actions, you part gradually from the "self":

"It is your problem, why me praying for you, for your karma?"
"Hey, I will try my best, but you gonna do your part"

Once the above reactions are no longer present in your mind as you help, your Compassion has arisen purely and it is no longer about "I" and "you".
Through the experiences of helping others though Compassion, one may learn as well - one would learn how to observe unbiasedly over time, and because of this, one's help to others would become selfless and truly beneficial to those in need.
Not just because "I feel great and better as a person, because I help."
Because if we do, it would be about our ego. The help to others would be about us, ourselves.
Thanks for question.

I believe in talking with people in the vocabulary that they understand and what we do in metta meditation is like prayer, so I would say, "Of course, I will meditate for you."
Then in my compassion meditation I would include them in the appropriate circle in my compassion meditation

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

for the circles of:

myself
people I have positive feelings about
people I have neutral feelings about
people I have negative feelings about
everybody
all beings

I tell them that I will include them/the person in question in my practice.

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text:
What type of Buddhist monks are only allowed to eat two meals per day and why?
In this answer, user yuttadhammo described that monks are only permitted two meals per day during the morning hours, from dawn to noon.
What type of Buddhist monks are permitted only two meals per deal? Why are theseBuddhist monks only permitted two meals a day in this time period?

Does this apply to all Buddhist monks?  It may be necesary to narrow the scope of the question.

Hmmm. I will edit it.

Those monks who follow the pātimokkha (Sk. Prātimokṣa), either Mahayana or Theravada, keep this rule (or, in certain cases, a rule to eat only one meal per day). The rule is simply:

37. Should any bhikkhu chew or consume staple or non-staple food at the wrong time, it is to be confessed.

The wrong time is generally understood to mean outside of the morning hours between dawn and noon. So, technically, one could eat ten meals during this time if one so desired. The Buddha himself enjoined a single meal, as stated in the Bhaddāli Sutta (MN 65):

“Bhikkhus, I eat at a single session. By so doing, I am free from illness and affliction, and I enjoy lightness, strength, and a comfortable abiding. Come, bhikkhus, eat at a single session. By so doing, you too will be free from illness and affliction, and you will enjoy lightness, strength, and a comfortable abiding.”
(Bodhi, trans.)

Later, the monks were concerned about eating rice soup in the morning, thinking (I assume) that it would be considered a second meal. The Buddha enjoined them to partake of it, citing physical benefits (can't find the source right now, will try to add it later). So, it is generally understood that one should eat a single meal before noon, but may partake of something light at first light of dawn as well.
Many monks these days stretch or even break these rules, drinking milk, soya milk, etc. or even eating in the evening, but those who follow the rules, in both Mahayana and Theravada, will generally stick to one or two meals per day.
Eating once per day is actually specified as an optional (dhutanga) practice, meaning one would forgo even rice soup in the morning.

Actually, in Pali Canon, Buddha suggests to his students to only eat one time a day, before noon. Among the reasons he provided, the three I remember are:

Because eating required begging, not eating in the evening meant not begging at dark, which could be both dangerous for the beggar and, more importantly, scare the householders by generating all kinds of ambiguous situations (that could be interpreted as intention to steal, or rape, etc.) resulting in bad reputation for the sangha.
Apparently, some students were begging too much and then hid their food in holes they dug in the ground. This was promoting anti-sanitary conditions and cases of monks stealing each other's food. Eating once a day reduced the need to store food.
Not eating too much, especially in the evening, supposedly helped the students feel better both mentally and physically.

After Buddha's death, many of these rules, originally meant for the wondering mendicants, were carried over to monasteries.

The legend goes in such a way that Siddartha came from a position of abundance and instant gratification, then stopped eating altogether, until he nearly starved and was offered food. Realizing that neither way released him from suffering, he started the "middle way".
So food restriction is a form of the middle way: enough to survive, but not so much that it would cultivate gluttony.


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What practices are included in (daily lay) life for a Buddhist?
There are a number of Buddhist traditions, each with different teachings and practices. What are the daily practices lay people normally live by?
For instance, living by five or eight precepts, in the Theravada tradition, or just daily meditation practice.
How do the different Buddhist traditions compare, in terms of daily practices? What is common among different traditions, and what varies?
Revised question Since many people ask about choosing a tradition on Buddhism.SE, the purpose of this question is to gain clarity about the differences (and similarities) between the traditions. There appears to be a clinging to the known tradition(s) among many, and rejection of other traditions (as good or bad)
Here are a few examples. I am looking for some more insightful comments from those experienced with more than one tradition, since these are merely what Google provided:
Bhikkhu Kantipalo (1995) from the Theravada tradition:

Cultivation of calm and insight.
Practice giving (dana) according to their faith.
Make an effort to keep the precepts (sila) pure.
Develop the mind in meditation (bhavana)
Taking refuge in the Buddha, Dhamma and Sangha
Undertake the five precepts

By Kyabje Lama Zopa Rinpoche: (source offline when writing this, cached)

Taking refuge
Prostrations
Mandala offerings
Vajrasattva mantras
Samaya vajra
Dorje Khadro (burning offering)
Water bowl offerings
Guru Yoga
Making tsa-tsas

Venerable Thubthen Chodron highlights:

Affirmations upon waking up
Morning meditation
Cultivation of mindfulness

SGI members are encouraged to:

Believe in the teachings of Nichiren
Chant Nam-myoho-renge-kyo, as well as to explain Nichiren's teachings to others
Study and understand the Buddhist teachings

Found this about Zen practice on zenki.com (unsure of the authority, since I'm unfamiliar with Zen)

This sounds awfully broad to me. I think it would be fine to have multiple different questions like this (one for each school/sect/whatever, because even within a sect, you will have different people with different opinions), but as is, there are far too many possible answers for this question.

I dunno... It could be taken as asking for a comparison between traditions, in which case an expert on comparative Buddhism might have a proper answer. Or at least, maybe that's what this question should be.

Updated the question

"daily lay"? : )

I think this should be reopened as it is a very common and popular question.
For Buddhist lay people throughout history and in all Buddhist traditions there have been two main practices:

dana to the sangha (giving donations, supporting the sangha)
ethics, usually in the shape of an attempt to live as closely as possible by the 5 precepts.

As this question references mainly Mahayana sources, it does not belong in Theravada.

I think the threefold path is quite a useful model. So

Ethics - some form of ethical practice. Typically deriving from the 5 precepts
Meditation - i think that most practictioners would have a (semi) regular meditation practice
Wisdom - some continuing engagement with the Dharma whether self -study, study group or just an appreciation of the 4 noble truths and eightfold path

BUT
Practically membership of some kind of sangha. Maybe contraversial but without this I think it is difficult to maintain a practice. The other things would then flow from this.

Yes, it is the five precepts on normal days and 8 precepts on Poya days. Apart from that, do meditation and good deeds whenever there's an opportunity. Or simply come up with plans to avail opportunities to do good deeds. Just like how we normally plan or devise ways to gain sensual pleasures.

Hi Sankha. I have found this website: http://portalseven.com/calendar/Holidays_Sri_Lanka.jsp?year=2015 . I was wondering if one should keep the 8 precepts on the other days too or only on the Poya days?

You can keep to 8 precepts on other days as well, if you are capable

Ok thank you :)

It depends on the tradition. Theravada, Mahayana, Vajrayana and their myriad "flavors" recommend different practices for laypeople.
As a general rule, Crab Bucket's answer seems like a good place to start, including the part about joining a sangha of some kind.
The Pali Canon also includes the Sigalovada Sutta, where the Buddha gives some practical (though not geared towards reaching enlightenment) advice on how to lead a householder's life.


text:
Differences of Ultimate Realities in different traditions
In Theravada Abhidhamma Pitaka there are four Ultimate Realities (paramattha dhammaa) mentioned,

Citta
Cetasika
Rupa
Nibbana

Is there any variations in these realities in other traditions of Buddhism?

The Theravāda school has 1 citta (arising in 89 or 121 combinations), 52 cetasika, 28 rūpa and 1 unconditioned element (Nibbāna). The Sarvāstivāda school had 1 citta, 60 cetasika, 11 rūpa and 3 unconditioned elements. The Sautrāntika school had 6 citta, 29 cetasika, 8 rūpa and 1 unconditioned element. The Yogācāra school had 8 citta, 75 cetasika, 11 rūpa and 6 unconditioned elements. Each of these schools used the same set of Suttas, but analyzed them in a different way, according to their own doctrines.

Citta is consciousness, but citta never arises alone, always together with a collection of cetasika (mental factors). The Dhammasaṅgaṇi lists 89 combinations. These combinations are grouped according to create new kamma / result of past kamma / functional (unrelated to kamma). These combinations are also grouped according to unwholesome / ethically neutral / beautiful.
Among the 89 are 4 "path" and 4 "fruit". If we subdivide these according to the 5 jhāna, then we get 20 "path" and 20 "fruit" and the total increases from 89 to 121.

@Lanka, oops I may have accidentally deleted your question... but I retyped my answer above

There are those traditions that disregard pursuing an "ultimate reality." In Tools of Perception (and mp3), Thanissaro says:

There's a point where the perceptions have done their work and you put them aside. That's the correct use of all the different ideas and concepts that we pick up from the study of the dhamma. The incorrect use is to say there is an ultimate view of reality and there are conventional views of reality and what we're trying to do is get the ultimate view which describes things as they really are.
And so you latch on to those ideas: okay, this is ultimate reality and you hold on to it as if you could hold reality in words. But if you hold on to it with words, what are you doing with it?

His point being that even if you found an "ultimate" reality, what use is it compared to what your actions are? What use is having a mental description of something when the mind has not gone beyond suffering:

So we're not trying to get to an ultimate description of reality, we're trying to get to an end of suffering--two very different things.

I'm pretty sure that Thanissaro rejects the Abhidhamma, which is kind of a rejection of the Theravada, so...

Edited my post to avoid mentioning Theravada altogether.

I'm completely onboard with Thanissaro on this. I believe he presents what was Buddha's original position. Also, in Vajrayana, starting from Yogatantra level and up, there is no separation into ultimate and relative.

Since you asked about "other traditions" I could not go by without mentioning Prajna-Paramita, a movement within early Mahayana that emerged as a reaction against Abhidhammists (personified by Sariputta) allegedly reifying dhammas as ultimately real.
According to Prajna-Paramita, the view of dhammas as having independent self-existence (despite Buddha explicitly declaring anatta as a characteristic of all conditioned phenomena without exception!) is yet another hindrance in the mind, leading to fear, confusion, and preventing awakening to (self-existing, unconditioned) Nirvana.
Instead, the only ultimate reality is said to be Shunyata, unity of form and emptiness, an umbrella term for Three Characteristics of Existence plus Nirvana.

This is one of classifications of paramartha dharmas. In (most) expanded form they also known as dharma lists. There is many variations in these lists between traditions. Sometimes traditions differ in content (if some element should be included in the list or not), sometimes in status of elements in there. Probably all four categories have difference in some or another tradition.

In my opinion there should be no changes from school to school.
Conventional reality is what is within the field of Perception. Ultimate reality is things not comprehended or understood within the field of Perception.


text:
Does enlightenment mean achieving purity?
Usually,  the word enlightenment is not directly understandable; a newcomer might understand incorrectly.
In the Visuddhimagga (Path to Purification, p. 6), Nibbana is illustrated as follows:

"Herein,  purification should be understood as Nibbana, which being
  devoid of all stains, is utterly pure."

On page seven it is further explained as follows:

"Formations are all impermanent: When he sees thus with understanding
  And turns away from what is ill, That is the path to purity"

Are enlightenment and Nibbana the same?  If so, does enlightenment mean achieving purity?

I've taken the liberty of narrowing the focus of the question in the title more in line with the question. The nature of enlightenment is such a broad and fascinating question I feel it would be a shame not to try to tackle the facets of the issue individually. Hope that works for you - feel free to revert

There are four stages of enlightenment (as described in the Anapanasati Sutta), Sotapanna, Sakadagami, Anagami and Arahant; stream-enterer, once-returner, non-returner and Arahant; respectively.

Stream enterer: A stream-enterer reaches arahantship within seven rebirths upon opening the eye of the Dharma.

...with the wasting away of [the first] three fetters, are stream-winners, steadfast, never again destined for states of woe, headed for self-awakening...

Once-returner: Both the stream-enterer and the once-returner have abandoned the first three fetters. The stream-enterer and once-returner are distinguished by the fact that the once-returner has weakened lust, hate, and delusion to a greater degree.

...with the wasting away of [the first] three fetters, and with the attenuation of passion, aversion, & delusion, are once-returners, who — on returning only once more to this world — will make an ending to stress...

Non-returner: The non-returner, having overcome sensuality, does not return to the human world, or any unfortunate world lower than that, after death.

...with the wasting away of the five lower fetters, are due to be reborn [in the Pure Abodes], there to be totally unbound, destined never again to return from that world...

Arahant: The fourth stage is that of Arahant, a fully awakened person. He has abandoned all ten fetters and, upon death will never be reborn in any plane or world, having wholly escaped saṃsāra.

...who are arahants, whose mental effluents are ended, who have reached fulfillment, done the task, laid down the burden, attained the true goal, laid to waste the fetter of becoming, and who are released through right gnosis...

Individuals may experience nirvāna as an object of mental consciousness. Certain contemplations with nibbana as an object of samādhi lead to the level of non-returning. At that point of contemplation if the individual realizes that even that state is constructed and therefore impermanent, the fetters are destroyed, arahantship is attained, and nibbāna is realized.
With regard to nirvana, a distinction is made between a person's experience of nirvana during their life and after their death. These two aspects of nirvana are described as:

Nirvana during life (nirvana with remainder) - indicating the experience of someone who has experienced nirvana in their lifetime but still remains in their physical body.

Nirvana after death (nirvana without remainder) - indicating the experience of nirvana for someone after their death.

Initially, the practice does involve "purification" (of mind from mental and emotional toxins or stains or obscurations) but on advanced stages the dichotomy of purity-vs-impurity itself is considered an impurity and has to be "purified away".
Whether Enlightenment is Nirvana or not is a difficult technical question. According to one definition, Nirvana dependently-coarises with Samsara as its inverted projection, while full Enlightenment is transcending both.
For beginner it is Ok to assume that Nirvana and Enlightenment are the same and is achieved by purification, from attachments and preconceptions. Once you get closer it will become clearer what is what.


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How does making merit affect karma?
I believe I understand the basics of karma as the effects of our intentions and actions (good & evil) which will play some role in this and future lives. But how does merit making affect karma? If a person has done something evil in the past can making merit improve one's karma? Is merit gained even for insincere actions such as when people do something good but are motivated by gaining reputation or status? Or is merit not a real thing but just a concept which functions to encourage people to do more good deeds so they'll have less time or inclination to do bad deeds?

Long after I asked this question, @catpnosis passed along http://www.accesstoinsight.org/tipitaka/an/an07/an07.049.than.html in response to question http://buddhism.stackexchange.com/questions/2357/right-understanding-of-d%C4%81na. This sutta really answers some of my questions above as well. Thanks catpnosis! :)

This page from What Buddhists Believe, as pointed out by Dewmini Gunasekera below, is a great read on this issue.

I have heard a simile which give some insight around this question. It goes like this. Suppose that you have a big bag with black and white pebbles. The black pebbles are the bad "Karma" and white are the good "Karma" which you have done in your past. When you do an act of merit, you add one white pebble to your bag. Add a black pebble for unwholesome acts.
So if you can fill the bag with lot of white pebbles (wholesome acts), there is a high chance that you would get a white pebble (good karma) from your bag. In other words you import your good karma by doing the good acts.
I think when it comes to karma, the basic idea is "chetana" is the most important aspect. I hope you have heard the quote "chetanaham bhikkawe kamman wadami". So with the insincere actions, there can be some good karma from the action, but as the "chetana" is not pure, the power of karma is lesser, according to my understanding.

Kamma is action which produces results, which could be called "merit." It's like seeds planted in a field, no one knows for sure when they will ripen, but if they were planted, they will ripen:

Just as when seeds are not broken, not rotten, not damaged by wind & heat, capable of sprouting, well-buried, planted in well-prepared soil, and the rain-god would offer good streams of rain.
  Those seeds would thus come to growth, increase, & abundance.
In the same way, any action performed with greed ... performed with aversion ... performed with delusion — born of delusion, caused by delusion, originating from delusion: wherever one's selfhood turns up, there that action will ripen.
  Where that action ripens, there one will experience its fruit, either in this very life that has arisen or further along in the sequence.
AN 3.33

Insincere actions are those performed with greed or delusion. There's no "tricking" kamma; if you do a greedy action, the consequences are automatic, like how a person can't indulge in sensual pleasures without also becoming accustomed and attached to them in the process.
If evil has been done in the past, performing good actions now doesn't cause it to become unplanted, but developing good qualities of mind here and now means that when things ripen, you can lessen their effect.
For example, someone who meditates and keeps in mind the impermanence of all things will be less likely to be driven into rage and depression if a flood comes in and destroys their home, because this is a perfect example of that impermanence they keep reminding themselves of. They might even see the positive side that it re-affirms to them the principle of impermanence. If you don't develop good kamma right now by developing the mind, you're more likely to lose it when bad things happen.

When I asked this question, I did not have a good understanding of how fundamental making merit is to Buddhist practice. So it was interesting to look further into it myself.

Of all the concepts central to Buddhism, merit (puñña) is one of the least known and least appreciated in the West. This is perhaps because the pursuit of merit seems to be a lowly practice, focused on getting and "selfing," whereas higher Buddhist practice focuses on letting go, particularly of any sense of self. Because we in the West often feel pressed for time, we don't want to waste our time on lowly practices, and instead want to go straight to the higher levels. Yet the Buddha repeatedly warns that the higher levels cannot be practiced in a stable manner unless they develop on a strong foundation. The pursuit of merit provides that foundation. To paraphrase a modern Buddhist psychologist, one cannot wisely let go of one's sense of self until one has developed a wise sense of self. The pursuit of merit is the Buddhist way to develop a wise sense of self.

Merit: A Study Guide by Thanissaro Bhikkhu (quoted above) lists three activities that lead to merit; dana (giving), sila (virtue), and bhavana (meditation). This is followed by around 71 pages of sutta passages, dhammapada verses and notes that mention various aspects of merit and how important it is.
So my original question which was cynical and hinting at the fact that our motives for doing anything are rarely pure, is simply overshadowed by the fact that making merit is clearly important and holds great benefit for practitioners.

Sometimes merits can cancel out bad kamma, known as Ahosi Kamma. Yes good karma can cancel out some of bad karma. Good deeds with the seek for reputation, as far as I know, if someone does something with a good heart but expecting reputation as another result, it can be still merit. Because someone has to get to nirvana to stop collecting merits or kamma. Merits is a real concept. When someone does good deed even without affiliation for reputation, he will get it very soon, in this life itself.
Think of someone who gives alms to an arahat, he will receive the benefits this life itself or in the next and good results will come until he reaches Nirvana. The mind of the person who accepts alms also matters, the power of the will of the giver also matters. And Dana sila bawana are only the three main methods to get merit. There are ten methods known as Dasa Punya Kriya. Those three are the first three under the ten methods. Just search for "Ten meritorious acts".
You made a question to clear and make your view correct. When you understand the answers for the question, u collect merit. It's the number ten in the meritorious acts. And I am trying to teach you something I know on Dhamma, it is also a merit, number eight in the list.
However I will mention the ten

Dana
Sila
Bawana
Reverence
Services in helping others
Transferring merits
Rejoicing merits
Preaching dhamma
Listening to Dhamma
Straightening views

Merits a good foundation for higher levels. Without merits, you cannot continue.

Thank you for pointing out the Ten Meritorious Actions. That was good information.

You are welcome!


text:
Is it only humans who can understand Buddhism?
If Buddhism is a general theory of understanding the cycle of attachments, is it possible that any other 'wise' enough creature could achieve the same thing?
In the Visuddhimagga English translation,  a creature who can understand this theory is called 'a wise man'.
In the Visuddhimagga Sinhalese version,  a creature who can understand this theory is called as a creature with 'Trihethuka Pratisandi'. i.e. Tri (three) + hethuka (caused by factors) + Pratisandi (birth): 'A birth caused by three factors' (Pali: tihetukapa.tisandhi).

Yes, humans and also meditating chickens: "Some people think that the longer you can sit, the wiser you must be. I have seen chickens sit on their nests for days on end!" - http://www.buddhanet.net/bodhiny2.htm

Somehow this question made me wonder whether the dharma is expounded in a pure land.

Actually, the restriction has nothing to do with being a human; it has, as the Visuddhimagga notes, everything to do with the rebirth-linking consciousness (patisandhi). Nor does it relate to "understanding Buddhism" per se, but rather attainment of samatha jhana and nibbana. Only a being who is reborn with all three of the wholesome roots - non-delusion (amoha), non-greed (alobha), and non-anger (adosa) can attain absorption (jhana), the path (magga) or fruition (phala).
Types of beings that may be born with these three roots are humans, angels (deva) and gods (brahma). Animals, ghosts and hell beings are not.
For more information, here's a good summary of the types of rebirth based on number of roots:
http://www.wisdomlib.org/buddhism/book/abhidhamma-in-daily-life_2/d/doc3181.html

But we don't know the existence of angels and gods. Do we?

@kalan I don't see how that's relevant...

this answer does not appear to conform with SN 56.114-131, which appear to state hell-beings, ghosts & animals can realise the four noble truths.

Only humans and the beings in the higher realms can attain enlightenment. Except for the ones in certain Brahma realms.  Some beings in the lower realms may hear the Dhamma, memorise it and be born in higher realms as a result.

To be pedantic, any sources?

Yes, you need to be human.
The Buddha even made the famous simile of the turtle in the ocean that comes up for air once every century and, when it does, happens to put its neck through a yoke floating around in the same ocean. He said that that's how lucky you are for having been born a human, so you shouldn't waste this opportunity to work towards enlightenment.
If you're another kind of sentient being, you'd better hope that you're reborn as a human in your next incarnation.

This is not according to any Buddhism I know... I think you forgot about angels (Deva).

@yuttadhammo : Please see the Chiggala Sutta in the Tipitaka, Samyutta Nikaya 56.
Link: http://www.accesstoinsight.org/tipitaka/sn/sn56/sn56.048.than.html

Right, that part is fine... it's the part where you answer that you need to be human to understand dhamma. Devas and some brahmas can as well.

@CarlosAccioly Do you answer from a Theravada viewpoint?

@rem Well, I used to follow the Theravada path, but nowadays I'm kind of nondenominational. The Buddha said, "place no head above your own," so I take what I think makes sense from each branch of Buddhism... and from other religions as well.

Kôdô Sawaki said:

"All creatures are Buddha already, but it is the human alone which
  needs to express his Buddhaship every day".

I don't know if this originate in a teaching of Gautama Buddha or if it is Sawakis interpretation alone. However I understand it like all humans are Buddha by default, as all other animals. But humans might "lose their way" and so its their practice alone to return to the Buddhaship.
That for me also means that animals do not need to understand Buddhism, because they are already Buddha. Understanding also would mean to try thinking about it. I doubt animals do that, and if they could, I doubt they see any benefit of trying to understand anything which is "just there".
We humans try to understand Buddhaship, as we can loose this state of mind. Bodhidharma once said, we do no need to understand Buddhism, because we would never understand as long as we want. Instead we just need to practice Buddhism, which will make you understand.
(I am practicing Zen, thats why my response might be from a view from the Zen angle)

Perhaps the question needs to be clearer. Are you talking of Buddhism as in the teachings of the Buddha or Buddhism as taught by the two main branches (Mahayana and Theravada) which could be said to be the interpretation of the teachings of the Buddha, often blended with cultural influence?
I have a pet corn snake and she could be considered to be in a mindful state for much of the time - she neither cares about the past nor fears for the future. She hunts when she's hungry and rests when there is no need for her to move. She never appears to be dissatisfied, she has no attachment to physical things and she does not crave anything beyond her basic needs for survival. By that definition, has she not already achieved Nirvana?
The challenge to human enlightenment is that we do crave, we do form attachments to experiences and possessions and we constantly experience dissatisfaction with our life experience. Can an animal that exemplifies the four noble truths be said to understand Buddhism? Not really - they are as they are - they have no need for Buddhism nor an understanding of it.

Even the pet corn snake is not safe from illness or death.

True - but the snake does not worry about it either.

Maybe only a Buddhist can understand Buddhism.
But enlightenment, do you think that an Asian have more chances to reach enlightenment than a European?
Anything based in the impermanence of our senses and mind will produce more ignorance.
This is also ignorant but I prefer lo live in a world where the enlightenment it is reachable from any living or not living form, and right here, right now.

There is a series of suttas at the very end of the Samyutta Nikaya, namely, SN 56.114 to SN 56.131, which appear to state 'hell beings', 'animals' & 'ghosts' can realise the Four Noble Truths.

I'm assuming that by "understand Buddhism" you mean achieving arahantship.
The Buddha is known as the "teacher of humans and devas" so at least both humans and devas.
In many suttas The Buddha gives teachings and instructions to devas.
Devas seem to be just like humans basically like extraterrestials, except most are longer-lived, experience more pleasure, are better looking with better brains, better senses, etc...many are possessed of iddhi.
It is also likely that others like nagas, asuras, yakkhas, and other beings intelligent enough would be able to understand Buddhism because Maha-samaya Sutta (DN 20) mentions some of them as having iddhi power and visiting and taking refuge in The Buddha.
Nagas seem to be humanoid reptile-like beings.
But in general most humans (and other beings in the universe) won't be able to understand Buddhism well enough to achieve arahantship and some are actually incapable.

"Monks, one who hasn't abandoned nine things is incapable of realizing
  arahantship. Which nine? Passion, aversion, delusion, anger,
  resentment, arrogance, insolence, envy, & stinginess. One who hasn't
  abandoned these nine things is incapable of realizing arahantship."
  (AN 9.62)

You could say that only arahants, paccekabuddhas, and sammasambuddhas understand Buddhism.
Also even beings that are willing and capable of achieving arahantship still won't, The Buddha uses similes of attempting to make a fire with wet wood:

"Then these three similes — spontaneous, never before heard — appeared
  to me. Suppose there were a wet, sappy piece of timber lying in the
  water, and a man were to come along with an upper fire-stick,
  thinking, 'I'll light a fire. I'll produce heat.' Now what do you
  think? Would he be able to light a fire and produce heat by rubbing
  the upper fire-stick in the wet, sappy timber lying in the water?"
"No, Master Gotama. Why is that? Because the timber is wet & sappy,
  and besides it is lying in the water. Eventually the man would reap
  only his share of weariness & disappointment."
"So it is with any brahman or contemplative who does not live
  withdrawn from sensuality in body & mind, and whose desire,
  infatuation, urge, thirst, & fever for sensuality is not relinquished
  & stilled within him: Whether or not he feels painful, racking,
  piercing feelings due to his striving [for Awakening], he is incapable
  of knowledge, vision, & unexcelled self-awakening. This was the first
  simile — spontaneous, never before heard — that appeared to me.
"Then a second simile — spontaneous, never before heard — appeared to
  me. Suppose there were a wet, sappy piece of timber lying on land far
  from water, and a man were to come along with an upper fire-stick,
  thinking, 'I'll light a fire. I'll produce heat.' Now what do you
  think? Would he be able to light a fire and produce heat by rubbing
  the upper fire-stick in the wet, sappy timber lying on land far from
  water?"
"No, Master Gotama. Why is that? Because the timber is wet & sappy,
  even though it is lying on land far from water. Eventually the man
  would reap only his share of weariness & disappointment."
"So it is with any brahman or contemplative who lives withdrawn from
  sensuality in body only, but whose desire, infatuation, urge, thirst,
  & fever for sensuality is not relinquished & stilled within him:
  Whether or not he feels painful, racking, piercing feelings due to his
  striving, he is incapable of knowledge, vision, & unexcelled
  self-awakening. This was the second simile — spontaneous, never before
  heard — that appeared to me.
"Then a third simile — spontaneous, never before heard — appeared to
  me. Suppose there were a dry, sapless piece of timber lying on land
  far from water, and a man were to come along with an upper fire-stick,
  thinking, 'I'll light a fire. I'll produce heat.' Now what do you
  think? Would he be able to light a fire and produce heat by rubbing
  the upper fire-stick in the dry, sapless timber lying on land?"
"Yes, Master Gotama. Why is that? Because the timber is dry & sapless,
  and besides it is lying on land far from water."
"So it is with any brahman or contemplative who lives withdrawn from
  sensuality in body & mind, and whose desire, infatuation, urge,
  thirst, & fever for sensuality is relinquished & stilled within him:
  Whether or not he feels painful, racking, piercing feelings due to his
  striving, he is capable of knowledge, vision, & unexcelled
  self-awakening. This was the third simile — spontaneous, never before
  heard — that appeared to me." (Maha-Saccaka Sutta, MN 36)

No matter how hard someone tries to make a fire even if they are willing and capable if their method is wrong they won't succeed.
In the same way no matter how hard someone tries to achieve arahantship even if they are willing and capable if their method is wrong they won't succeed.
It is doubtful that there any arahants or paccekabuddhas in the world right now. Even during The Buddha's time with his assistance and teaching there were only a few thousand arahants in the world.
Since you could say that only arahants, paccekabuddhas, and sammasambuddhas understand Buddhism you could say that it is doubtful that anyone in the world right now understands Buddhism.
There probably are however some humans in this world who have achieved higher states (jhanas) and developed iddhi powers (concentration) but still haven't achieved arahantship (the ending of mental fermentations), and also many who mistakenly believe themselves to be enlightened.

I don't fully understand Buddhism, so I'm not an arahant, paccekabuddha, or sammasambuddha. But I feel that I'm on the path towards becoming a paccekabuddha. I think that I have achieved at least the 1st jhana, but my concentration is just ok. Now while waking I can enter higher states in 5 seconds or less.
Since the exact method and process of becoming an arahant seems to have disappeared achieving arahantship in modern times is like the same as becoming a paccekabuddha.
The distinction between arahants (that have the six higher knowledges), paccekabuddhas, and sammasambuddhas is small


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What is the difference between relative and absolute truth in Buddhist philosophy?
Many Buddhist texts and teachings make a distinction between absolute and relative truth. Please explain the difference and give some examples.

Wikipedia has an article about it that is terribly difficult to read and comprehend. I'm eager to see what this community can do with it :)

also see http://buddhism.stackexchange.com/questions/11625/conventional-versus-ultimate

In Theravada, as the Wikipedia article states, there are types of truth: conventional truth (sammuti-sacca) and ultimate truth (paramattha-sacca).
These two types of truth are not exactly the same as relative and absolute truths. Rather than focusing on the universality or specificity of the scope of the truth, they focus on the intrinsicness of the truth - i.e., whether an object agrees with a truth by its inherent nature, or whether it is simply said to agree with it by external convention.
An example of conventional, extrinsic truth is in a fist. When you close your hand, a fist appears, almost by magic. When you open your hand again, the fist disappears. There was never anything intrinsic about the object that made the fist appear; only convention and recognition gave rise to the truth or falsehood of the arising of a fist.
An example of ultimate, intrinsic truth is in the pain felt when a fist connects at high velocity with a face. The fist and the face are only conventional descriptions of the reality, but the pain is intrinsic to the experience and thus said to be ultimately real. Also ultimately real are the vision of the fist coming towards the face, the feeling of the hardness of the fist and the softness of the face, and the sound of the fist connecting with the face.
In brief, conventional reality refers to entities, while ultimate reality refers to the building blocks that make up reality (i.e. experience). So, the entities of fists and faces are conventional, but the sights, sounds, and feelings associated with the experience are ultimate reality.
The importance of this doctrine is in establishing a proper paradigm for understanding reality. Because conventional truths are based on mental formations, they cannot aid in an understanding of reality. A permanent, satisfying, controllable convention can be conceived of, but it will never exist in ultimate reality. By delineating that which exists in ultimate reality, one can limit one's observation to those objects that will provide an understanding of how reality works.
For example, focusing on the concept of food as you eat, you will never come to see the nature of reality as arising and ceasing incessantly, because you are focused on something that is constant (but not really existing) - i.e. the food. As a result, you will easily conceive of it as good or bad, me and mine. If you focus on the experience of tasting and feeling as you chew the food, you will notice that you don't actually taste or feel the food always, your mind flitting between tasting, feeling, liking, disliking, thinking, etc., and you will come to understand the nature of reality, which in turn will allow you to let go of your attachments to good an bad experiences as you see that there is nothing truly desirable or undesirable about them.
This is based on the teaching of the Buddha while he was alive.
Relative and absolute truth, I believe, are more a part of the Mahayana claim that the teachings of the Buddha during his life were only meant to be taken in the context of the audience's limited ability to understand higher truths. Such teachings, it is claimed, can therefore be disregarded or at least superseded by later teachings that were kept secret until a time when beings could understand the higher truths they contained.
So, an example of a relative truth would be the teaching of dependent origination and the four noble truths; ultimate truth would be seeing that even those teachings are emptiness, and therefore meaningless. The point is that in order to understand the ultimate truth of emptiness, one must first realize the conventional truths as stepping stones.
I'm sorry if I haven't done the Mahayana view justice, but I don't subscribe to it myself; there is nothing in the original teachings of the Buddha to suggest that they are only relatively true, or that there is anything absolute beyond them. But, I guess I'm biased :)

+1 for acknowledging you could be biased :)

As @Yuttadhammo pointed out, relative and absolute truths are a Mahayana construct. In Pali Canon, Buddha does not declare such duality (all he does is caution his students against taking his metaphors literally).
From Mahayana standpoint, the phenomenal world, consisting of subject and its objects as well as relationships between them, is relative truth. Any description of reality is necessarily in the realm of relative truth as it depends on arbitrary axioms posited against each other. The ultimate truth ("what really IS") is therefore ineffable, but can be tapped in practice through deconstructing all (relative) constructs.
The difference between Relative and Ultimate truths becomes important for understanding Mahayana concept of Enlightenment:
From Mahayana perspective, the Path to Nirvana an individual walks belongs firmly in Relative Truth. However, because ultimately no separate "individual" exists apart from the interacting mental-and-physical factors, there is no entity that achieves Nirvana. Instead, when Three Marks of existence are fully realized as Emptiness of Interbeing, the notion of Nirvana is no longer applicable and Self-Existing Enlightenment can finally be realized.
Reality is an interpretation we make. Our everyday world, with its "I" and "others", birth and death is one such interpretation. The world of Buddhist practitioners, with its skandhas, samskaras, dharmas, and nirvana is another interpretation, adopted temporarily as a stepping stone from the first. Regardless of where one is on the Path, Ultimate Truth remains the ground reality underlying all worlds.

Aren't these terms semantics? If relative or conventional are constructs of the mind, it isn't clear if the intrinsic truth is simply the experiences of these constructs e.g. fist hitting the face since both the fist and face are constructs of the mind as is the pain. If so, what is intrinsic truth?

Right, right - but the emphasis here is that there are "three visions" as Sakya school calls them - not two.

How do these relate to the types of truth?

They are relative, semblance of ultimate or provisional ultimate, and the actual thing.

When you say they,do you mean the types of truth of the Sakya school? Is the actual thing ultimate truth?

"They" are three visions or three perspectives. How reality looks to an untrained person, to a Buddhist practitioner, and to an enlightened Buddha. Technically it's a different context of discussion, orthogonal to the question of truths in Madhyamaka, but in practice it all correlates.

Two truths is not just Mahayana development, it's Abhidharma (Abhidhamma) development, and exists in Theravada too (check this (pdf) excellent article of prof. Karunadasa). But "relative and absolute" is really misnomer, properly said it's conventional and ultimate truths (skt. samvriti & paramartha satya). Their distinction is like between naïve language (convention) and scientific analytical discourse. When language is used in its common sense it's conventional, and when it express Buddhist teaching it's truth. When reality as it is, is analysed into dharmas and expressed in special dharma-discourse (adhidharma), it is ultimate truth.
Typical example is: when Buddhist teaching talks about beings it's conventional truth, when it talk about skandhas it's ultimate truth.

The centerpiece of his Madhyamaka or ‘‘middle way’’ philosophy is the thesis that everything is empty. This thesis has a profound consequence. Ultimate truths are those about ultimate reality. But since everything is empty, there is no ultimate reality. There are, therefore, no ultimate truths. We can get at the same conclusion another way. To express anything in language is to express truth that depends on language, and so this cannot be an expression of the way things are ultimately. All truths, then, are merely conventional.
This is what Jay Garfield and Graham Priest calls "Nagarjuna’s first limit contradiction." More on this in Nagarjuna and the limits og thought" http://www.thezensite.com/ZenEssays/Nagarjuna/NagarjunaTheLimitsOfThought.pdf
Narjuna on the relation between the two truths: The notion of ultimate truth (reality) (paramārtha-satya, literally ‘‘truth of the highest meaning,’’ or ‘‘truth of the highest object’’) can be defined negatively as the way things are, considered independently of convention, or positively as the way things are, when understood by a fully enlightened being who does not mistake what is really conventional for something that belongs to the very nature of things.
What is ultimate truth/reality, according to Nāgarjuna? To understand this, we have to understand the notion of emptiness, which for Nāgarjuna is emphatically not nonexistence but, rather, interdependent existence. For something to have an essence (Tibetan: rang bzhin; Sanskrit: svabhava) is for it to be what it is, in and ofitself, independently of all other things. (This entails, incidentally, that things that are essentially so are eternally so; for if they started to be, or ceased to be, then their so being would depend on other things, such as time.)
To be empty is precisely to have no essence, in this sense. The most important ultimate truth, according to Nagarjuna, is that everything is empty. Much of the Mulamadhyamakakarika consists, in fact, of an extended set of arguments to the effect that everything that one might take to be an essence is, in fact, not so—that everything is empty of essence and of independent identity.
Dependent arising, relative truth, is emptiness and emptiness is the ultimate truth.
This realtion can be expressed with a slogan, namely "The Ultimate Truth Is That There Is No Ultimate Truth" (Mark Siderits )

I'm a Vajrayana practitioner and this is how the two truths were explained to me in very simplified terms.
On a relative level, which describes our every day experience, dualistic and conceptual thinking prevails and one feels separated from others. On the absolute level, however, there is no duality, and  object, subject and action are the parts of the same totality. On the absolute level everything is empty of intrinsic existence. This should not be confused with nihilistic thinking that things are hollow or not-existent - they do appear but they are interdependent and have no lasting properties. A great Indian Yogi Saraha phrased that in the following way:

People who think that things are real are as stupid as a cow;
People who think things are not real are even more stupid.

Understanding emptiness and thus the absolute truth is difficult and that is why the relative truth is employed to learn about the nature of mind. On the absolute level, there is exactly the same Buddhahood in a sacred temple as in a bathroom, but because of our habitual likes and dislikes, our meditation is deeper in a temple. A highly realised Lama has the same Buddha Nature as a criminal but on a relative level there is much more benefit to trust the Lama.
Once the emptiness is understood, the veils of ignorance disappear and a great joy is being discharged. Understanding the nature of the mind is indeed the lasting happiness and there is no greater joy than that. The following short poem nicely captures that:

Since everything is but an illusion,
Perfect in being what it is,
Having nothing to do with good or bad,
Acceptance or rejection,
One might as well burst out laughing!
Longchen Rabjam (1308-1363)

The concept of relative/absolute truth in early Buddhism is pretty straight-forward. Ven. Bodhi's note from his "Middle length Discourses" says:

The Buddha has a twofold teaching(2 levels of truth)-a conventional teaching/sammutidesana expressed in terms of persons, beings, women, men,etc and an ultimate teaching/paramatthadesana expressed solely in terms that possess ultimate ontological validity, such as aggregates, elements, sense bases, impermanent, suffering, not self,etc. The Buddha expounds his teaching thru whichever approach is best suited to enable the hearer to penetrate the meaning, dispel delusion, and achieve distinction..

For further info., also see Ven. Nyanatiloka's definition of paramattha from his "Manual of Buddhist Terms and Doctrines" here

The separator, between them, are causes (to arising), and imagination.
Ultimate truths are not imagination, because they are effects that arise by their own causes. Except nibbana, ultimate truth, is  the opposite of the other causes and effects, because nibbana is the cause of finshing of all causes and effects.
Conventional truths are imagination. They never arise and vanish. They just become to be the objects of consciousness, never arise.
Consciousness imagine conventional truth objects by some ultimate truths and some conventional truths, that it had known before. So the conventional truths never arise in real situation. But ultimate truths arise  in real situation by their own causes, because they are effects, not just imagination.
The example already available in the ancient canon:
The 1st-7th and first half of 8th chapters of abhidhammatthasangaha describe Ultimate truths.
The  last half of 8th chapter of abhidhammatthasangaha describe Consciousness truths.
http://www.palikanon.com/english/sangaha/sangaha.htm

Absolute truth (pure truth) is free of its opposite which is falsehood. Whereas relative truth is always attached to falsehood like the two sides of a coin. Relative truth is the always a result (an effect) of a cause; where as, absolute truth is never a result (effect) of anything and this is the reason why there is no path to Nirvana! Because defining a path means absolute truth (Nirvana) suddenly becomes the result (effect) of taking that particular path.