Is there a FAQ somewhere?
The basic idea: You’ve got a town, populated mostly by good folks, but with a few scum thrown in (mafia members, werewolves, Cylons, undead, whatever). The scum all know who each other are, but the innocent townmembers don’t know who anyone is. Every day, the townsfolk (all of them, including the scum lurking in their midst) vote for someone to lynch, and every night, the scum vote amongst themselves for someone to kill. At the start of the game, most of the players are good townsfolk, with a small number (a quarter or so) scum. The good side’s goal is to kill off all of the scum, and the scum’s goal is to reach at least half of the living population (this is as good as killing everyone, because from that point, the scum could just lynch everyone else). Whenever anyone dies, the game moderator announces to everyone what that player was.
In the basic version of the game, that’s all there is: The good guys have to find the scum based just on things like voting patterns (and luck, of course): If there are exactly four scum, and exactly four people all vote to lynch the same person without giving good reasons, and that person turns out to be an innocent Townie, then you’ve got a pretty good idea of who the scum are (and therefore, the scum would never do anything that blatant, and therefore, the Town has to look for more subtle clues, etc.).
Often, though, there are other special roles in play, who have powers they can use. Typically, the roles start off secret, known only to the person who has the role. The most common of these are the Detective (or Seer, in a supernatural-themed game), who every turn can investigate a player of their choice and determine what they are, and the Doctor, who can choose a player to protect every turn, and if that player is targeted by the Scum, they don’t die after all. The Detective is of course free to share his findings with the Town, but then again, a mafia member is also free to claim that he’s the Detective, and share false findings with the town, so good luck getting people to believe you.
I see, thanks for the explanation.
There’s also this:
http://wiki.flyingcowofdoom.com/mafiaWiki/Main_Page
And what the heck, since Idle won’t plug his own board, a bunch of us play slightly larger more complex versions of mafia over here:
http://psychopathgame.proboards106.com/index.cgi
Check it out. We are friendly.
Mafia games are very popular, like the tulip poplar, my favorite Western Hemisphere representative of the two-species Liriodendron genus.
There is also a Facebook -group
And we’re not only friendly - we also have sexy Mods 
Well, friendly in a “we’re going to lob all kinds of false accusations your way in an attempt to have you killed” kind of way. But really, when it comes down to it, that’s pretty much the best kind.
Chronos outlined the basic game well. But on top of the rules is the social game–players try to outguess each other’s motivations. (In a way, it’s like poker and bluffing.) A Scum player who’s a skilled actor could conceivably look exactly like a Town player. But it’s hard to do, especially under pressure. So these games can get very heated and acrimonious, as players try to find cracks in each other’s behavior. It looks harsh, but we’re friends and don’t hold grudges (some players do need to take occasional breaks).
Try reading one of the Mafia threads. I recommend a simple game (not one of mine), maybe Werewolf Mafia–The Split and it’s corresponding “forbidden” thread, where dead players and onlookers talk about the game. This will give you a feel for how the games go.
While you can read about it, the only way to truly understand is to play it. Keep an eye out for new game announcements (here or on one of the other places) and sign up to play. Introduce yourself and say you’re new to Mafia. Most experienced players will cut newbies some slack (and Scum rarely kill new players on the first few turns). Don’t expect to be a great player; even those of us who’ve played a lot make mistakes. And there’s not always a “right” way to play; there’s strong disagreement about how to do some things. Just take it in stride and try to do for your team the best.
Speak for yourself.
Ok, check it out, many of us are friendly and some of us will rip your heads off and defecate in the gapping wound. But there are more friendly ones than not.
WHAT??? You’re not friendly anymore???
Then from now on I’ll stick with the friendly NAF!