Stands for Team Fortress Classic, an online gaming mod for Half-Life. Teams are typically made up of players from nine classes (scout, sniper, soldier, demo man, heavy-weapons guy, pyro, medic, engineer and spy), and are pitted against each other in various missions, from capture-the-flag to eliminate-the-hunted.
Everything about the game is ancient, yet its replayability, fun factor, and absolute insanity are undeniable. TFC is a refreshing change from such 'serious' mods as Counter-Strike and Day of Defeat.
I stayed up till 2:00 AM last night playing Murderball on Team Fortress. Fun times.