"Trouble in Terrorist Town (TTT) is a gamemode included with Garry's Mod 10. The game is about a group of "terrorists" who have traitors among them, out to kill everyone who's not a traitor.
A small number of players is selected as Traitors, who have to kill all the Innocent players (ie. the rest of the players). Those innocents know they are in the majority, but they do not know who is Traitor and who is not.
The Traitors must use the element of surprise and their special equipment, if they are to succeed. The Innocent just have to survive, which means finding out who the Traitors are and killing them before they kill you. Of course everyone is holding a big gun, and everyone looks suspicious...
For the Innocent, knowledge is power: who is acting strangely? Who can be linked to evidence found on corpses? Who is still alive, even?"