As originally envisioned, Halo would have had internet capability. However, for various reasons, including the fact that the programming libraries necessary for utilizing the online features of the Xbox were not ready in time for the console's launch, the Xbox release version of Halo will not have online play as such.
In cooperative mode, two players using the same Xbox console can play through the solo campaign in a split-screen mode.
Up to sixteen players can play the other multiplayer modes using up to four connected Xboxes. Up to four players can use a single Xbox with enough controllers and using a split-screen mode. Any combination of players is possible: a total of 8 players, 4 each on 2 Xboxes, or 2 each on 4 boxes... you get the idea.
Halo's multiplayer modes apparently feature deathmatch, capture the flag, king of the hill, and others.