!! First Person Shooter (FPS)

A type of game where the game is played from the perspective of the main character. Obviously, shooting is usually involved. The FPS could not be implemented until computers reached sufficient power to draw a 3D (ish) environment in real time.

Castle Wolfenstien pretty much invented the genre, and was quickly followed by refinements on the idea, Doom, Doom II, Quake, Quake II, Quake 3 Arena, Unreal and Unreal Tournament, Duke Nukem 3D, Return to Castle Wolfenstien, No One Lives Forever, etc.

Half-Life rejuvenated the genre when it looked like the game playing public was over FPSes by adding a decent plotline to its single player mode.

Of course, online multiplayer and FPSes go together like two things that go together very well. So well that the latest version of Quake, Quake 3 Arena, ditched the single player mode of its predecessors entirely and became an online multiplayer game only.