At Time Magazine, they've got a quick and easy-to-understand primer on what Facebook is - and how it's different from MySpace and the rest of the internet. Here's the key pieces.
Facebook is, in Silicon Vall--ese, a "social network": a website for keeping track of your friends and sending them messages and sharing photos and doing all those other things that a good little Web 2.0 company is supposed to help you do. It was started by Harvard students in 2004 as a tool for meeting-- or at least discreetly ogling--other Harvard students, and it still has a reputation as a hangout for teenagers and the teenaged-at-heart. Which is ironic because Facebook is really about making the Web grow up.
Whereas Google is a brilliant technological hack, Facebook is primarily a feat of social engineering. (It wouldn't be a bad idea for Google to acquire Facebook, the way it snaffled YouTube, but it's almost certainly too late in the day for that. Yahoo! offered a billion for Facebook last year and was rebuffed.) Facebook's appeal is both obvious and rather subtle. It's a website, but in a sense, it's another version of the Internet itself: a Net within the Net, one that's everything the larger Net is not. Facebook is cleanly designed and has a classy, upmarket feel to it--a whiff of the Ivy League still clings. People tend to use their real names on Facebook. They also declare their sex, age, whereabouts, romantic status and institutional affiliations. Identity is not a performance or a toy on Facebook; it is a fixed and orderly fact. Nobody does anything secretly: a news feed constantly updates your friends on your activities. On Facebook, everybody knows you're a dog.
Maybe that's why Facebook's fastest-growing demographic consists of people 35 or older: they're refugees from the uncouth wider Web. Every community must negotiate the imperatives of individual freedom and collective social order, and Facebook constitutes a critical rebalancing of the Internet's founding vision of unfettered electronic liberty. Of course, it is possible to misbehave on Facebook--it's just self-defeating. Unlike the Internet, Facebook is structured around an opt-in philosophy; people have to consent to have contact with or even see others on the network. If you're annoying folks, you'll essentially cease to exist, as those you annoy drop you off the grid.
Facebook has taken steps this year to expand its functionality by allowing outside developers to create applications that integrate with its pages, which brings with it expanded opportunities for abuse. ... But it has also hung on doggedly to its core insight: that the most important function of a social network is connecting people and that its second most important function is keeping them apart.
And that's just it. There's no reason to "perform" on Facebook - since the only people that can "see" you on it are your friends.
Of course, the best way to understand Facebook is to just dive in, find your friends, and start communicating.