Introduction to the Mafia Game
Mafia was originally a party game, typically attributed to Dmitry Davidoff and cited as being created in 1986. The game models a conflict between a small group of informed players (the Mafia) and a large group of uninformed players (the Town) and was designed to analyze the psychology of players when placed in either group, especially as a study of the mob mentality. It made the jump to forums much later in mid-2000. The electronic medium allowed the game to evolve somewhat, featuring more complex setups and a slower pace that allows players to really consider their options, though it obviously lacks the necessity of a poker face found in the original form.
You can get a description of Mafia Game in Wikipedia and a Mafia Wiki.