First-person shooter

First-person shooters are a genre of video game. You play looking from the character's eyes, I.E. from the first-person perspective, whomever that may be.

History
Probably the first person shooter that brought it into the world was the game Battlezone, although the first game to make it truly popular was ID Software's Wolfenstein 3D.

ID Software's next game, Doom truly established the genre, with it's many evolutionary leaps forward, including the ability for players to move in three axes via stairs and the like, and with it's inclusion of multiplayer modes, allowing players to play with or against each other over a distance for the first time.

Halo and Halo 2 are also first-person shooters, although when you enter vehicles or turrets the view changes to third-person.

First-Person in Halo
The first Halo game introduced fairly few changes to the idea of a first person shooter. However Halo 2 introduced an interesting aspect never before seen: The character's own body. In most other first person shooters, when the character looks down they see only a shadow below them, as though they are a magical head and gun floating throughout the levels. However in Halo 2 when the character looks down, they see their own body. Characters are able to see themselves run, jump, and stand because their body exists in real-time during the game.