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.