This game is so much fun to play that I can't get it out of my system. So I thought I would bring it up here to see if we can scrap up enough serious players for a game:
In its simplest form, Mafia consists of two sides: the Mafia, and the Town. The Mafia's goal is to kill the innocents, while the Town's goal is to kill the Mafia.
At the beginning of the game, the Moderator will secretly send everyone their role. The Mafia will know each other, while (in the basic game) everyone else will only know their own role.
The game has two phases, the first of which is Day. During Day, the Town must choose someone to Lynch; they are trying to eliminate the Mafia, but the Mafia can lead them astray by casting suspicion elsewhere. Generally, players will vote for someone they think is Mafia (the Mafia will vote so that they seem to be doing this as well, and might even vote for each other to confuse the Town); when a player gets a majority of the votes, they are lynched. Their role is revealed by the Moderator, and it becomes Night.
During the Night, the thread is locked and no one can talk publicly. The Mafia, however, can speak to each other privately to plot strategy and decide who to murder before the next Day begins. In this way, they can silence members of the Town who are threats to them. At the end of the Night, the murder is carried out and another Day begins. In effect, the Town gets to kill someone, and then the Mafia gets to kill someone.
The game alternates between Day and Night until either the Mafia are eliminated, or the Mafia have killed everyone else (or nothing can prevent the same).
To see this another way, you may find this Flash introduction to Mafia by mikeburnfire (http://www.newgrounds.com/portal/view/610087) helpful.
The above suffices for a Vanilla game, but most games have roles other than just Mafia and Townie. These roles have additional abilities (which are usually used during the Night, though there are some Day abilities as well). Some more common roles are listed here.
Cop - Each Night, the Cop can investigate one player (by sending their name to the Moderator), and the Moderator will tell them whether that player is Mafia or not.
Doctor - Each Night, the Doctor can choose one player to protect (by sending their name to the Moderator). If the Mafia (or anyone else) tries to kill that player, that kill will be prevented.
Vigilante - Each Night, the Vigilante may kill another player (by sending their name to the Moderator).
Mason - The Masons are a group of players who are confirmed to each other as Town and can talk to each other at Night.
All of these roles are generally Town-aligned.
Serial Killer - The Serial Killer (often abbreviated as SK), like the Vigilante, may kill another player each Night (by sending their name to the Moderator). However, the Serial Killer is not on the Town's side or the Mafia's side. The Serial Killer wins when everyone else is dead.
Multiple Families - On occasion, in larger games there may be more than one group of Mafia. These act independently of one another, and must kill the other group as well as the Town in order to win.
Roleblocker - Each Night, the Roleblocker can choose a player. That player will be prevented from using their Night choice, if they have one. Roleblocker has been known to be a Town role as well as a scum role.
Anyone can play, even if you've never played before. If you might want to play, once we get enough players, feel free to say so in this thread!