Campaign

Summary
Normally referred to as a "Single Player" game, the campaign mode of Halo: Combat Evolved and Halo 2 would not accurately fill this description as it has a Co-op feature that allows two players to play the game simultaneously.

Campaign mode is where all canon from the games comes from. It is the place where the player undertakes the story of the game, meets all Characters, and fights computer-controlled enemies.

The other game mode of the Halo games is Multiplayer where there is no story line and the player plays against other humans.