Its important, I think, to note that this game is an adaptation of the 'party game' called Mafia. (link below)
Briefly, if you and 10 of your friends were to gather together, you could hand out cards - a few of them are Mafia, and the rest are Villagers. During the "Day" people discuss, and then they go around making accusations, and eventually take a vote. At night, everyone lowers their head, the Game Moderator says "Mafia, look up" and the Mafia players look up, silently point to someone they want to kill, and then put their heads back down. That player is dead and sits out the rest of the game, not saying anything. And so voting and killing continues.
The forum version changes a few things thanks mostly to the medium. Night and Day are merged, so that we don't have to have a weird stretch where no one can post.
The expansions are.. well its 2012, think of it as DLC or expansions. While I love the simplicity of Villagers vs. Werewolves, I think that Villagers + Specials vs. Multiple Werewolf Packs + Specials is just damn good fun.
http://en.wikipedia.org/wiki/Mafia_(party_game)
