It's a role playing murder game with only a few roles: the Honest People (majority), the Mafia (minority), and sometimes the Judges (moderators, one of whom is Mafia). It takes only a few hours. There are two kinds of turns, day and night, which alternate. No equipment is needed except a deck of cards. If you want to see some official rules, look here.
Mafia was first played as a regular event here on September 24, 1998, by a group of people who learned the game from our President, Steve Phelps, on September 15, 1998.