Details are pretty sketchy right now on what exactly this whole experience will entail. It will be a virtual world where you get to play as either a human, vampire, or werewolf -- that much is pretty known. The game will also be an open, non-linear world set around the town of Forks, Washington, that will let the player roam freely.
Of note is the functionality of the game's journal, which is said to work not only like a novel, but also as a way to teleport back and forth through time to experience the events written inside of it.
The game is still early in-development, so we'll keep an eye on it for you as more information is released. Now where's our Harry Potter MMO, hmmm? We want a wizarding war!
Update: Apparently, this MMO isn't as official as it seems.
[Via Twilight Source]