Trouble in terrorist town, is a classic game-mode seen in many games. It's about a group of “terrorists” who have traitors among them, who's job it is to kill everyone who is not a traitor.
When each round commences a random number of players are selected as Traitors, using they skills and equipment they must kill all the innocent players. The innocent usually out number the Terrorists 4 to 1 (assuming the server is running standard rules).
How to play
Simply join a game already running, or subscribe to the TTT game-mode. TTT can be played on any map that supports the spawning of weapons during rounds, but initially you're best playing with the dedicated TTT maps provided.