What is an LDS general conference? Tomorrow (Saturday) and Sunday, the Mormon church is holding a general conference, which happens twice a year, in place of regular church meetings.

There's some basic stuff on their website about it, but I'd like to know more about what it is, how it started, why it is held, its significance to church members, etc.

For example: It's a general conference -- who exactly is invited: is it for members only, like the temples? Is it a collection of sermons or does the format vary? What is discussed, and are there any notable conferences which have impacted Christianity or the world?

Bonus points for scriptural references or parallels.
I should probably let a member of the LDS Church answer this, as I only attended the LDS Church for a few months.  There was an LDS conference during that time, and I'm going to attempt to answer based on what I can find for official statements and my own memory of the event.

From the LDS Website:


  What is general conference?
  
  General conference is a semiannual gathering of The Church of Jesus
  Christ of Latter-day Saints. During general conference weekend, Church
  members and others gather worldwide in a series of two-hour sessions
  to receive inspiration and instruction from Church leaders.


Even though the main conference occurs in Salt Lake City, it's something that every member of the LDS Church can attend, in the local congregations.  In the one I was attending, it was shown over a projector, and all of the members attended.  They also use modern technology like Podcasts, YouTube, and others to give people that absolutely couldn't attend in person, or in their local place of gathering a chance to see and listen to the entire messages.  On top of that, the Deseret News provides extensive coverage.

The purpose is to give the leaders a chance to give guidance to the Church (as a whole - all members of the LDS Church, rather than local Churches), to preach, to exhort, and to instruct.  Also, announcements such as changes to Church Leadership, new direction, new understanding of doctrine based on divine revelation to Church Leadership is all common as well.

The teachings covered here are considered to be authoritative, and according to the following, are considered inspired.

Again, from the official LDS site: (bold text added by me)


  
  Read Doctrine and Covenants 68:2–4. What “scriptures” do we have today in addition to the standard works? (The inspired teachings of
  modern-day Apostles and Prophets.) Where can we find the teachings of
  modern-day Apostles and Prophets? (Church magazines and general
  conference reports.) How have you been blessed as you have treated
  their counsel as scripture?
  


For the members, it's a chance to hear directly from those highest in the Church leadership.

As much as I hate quoting from Wikipedia, it covers the basics quite well:


  General conferences are held on the weekends containing the first
  Sunday in April and the first Sunday in October. The April conference
  is known as the Annual General Conference, and the October conference
  as the Semiannual General Conference. The April conference includes
  annual statistical and financial reports not included in the October
  meeting. Both conferences are identified by the number of years since
  the church was founded in April 1830; thus, the April 2011 meeting was
  the 181st Annual General Conference, and the October 2011 meeting was
  the 181st Semiannual General Conference.


As for the significance to the Church members, aside from the obvious implications of the above:

It's not much different than any other annual or semi-annual business meeting for many secular businesses.  It's a chance to make big announcements, and motivation speeches, but of course, related to the Church and revelation from God, rather than the types of things important to businesses.

For someone who has never attended one, it's hard to describe on one hand.  To say that the Church Leaders give messages is a true statement, and they make announcements, and in general, give direction to the Church as a whole.  That all sounds a bit dry.  The official descriptions don't adequately describe the excitement and adoration that is evident.  It reminds me of how Apple employees reacted to Steve Jobs.  I don't mean that in a derogatory way.  I included it to attempt to convey the atmosphere, which is something I've never seen matched in other Church events. (Clarification - the meetings themselves are not like that - I was referring to the excitement of anticipation, the discussion in the car on the way to the Church, in the parking lot, etc.  Once the general sessions start, the atmosphere was much more reverent, but everyone gave rapt attention to the leaders.)

The excitement is understandable, considering that those they hear from include the President of the Church, and the Quorum of the Twelve Apostles.  Considering that the Church teaching is that these people are still receiving divine revelation from God, it's only one step removed from hearing from God Himself.

You can find much more information simply by searching their amazingly well done online Scripture Citation Index for "General Conference".  For example, from there I learned that Elder Paul V Johnson (one of the Seventy) considers it a sacred responsibility for all members of the Aaronic Priesthood to attend.


  It is a sacred responsibility to address you in this general
  priesthood meeting. I always look forward to attending these
  priesthood sessions with my sons. I have great memories of sitting by
  them in our stake center as we listened to the teachings of the
  General Authorities. These meetings made a difference in my life when
  I was a young man, and they continue to make a difference in my life
  now. I know they have been an influence on my sons and on millions of
  Aaronic Priesthood holders around the world.


And, not that it's doctrinally important, it's a chance to listen to the Mormon Tabernacle Choir, who are really quite good.