What is different between Halo and Halo 2?

According to Jason Jones of Bungie, Halo 2 is a lot like Halo only it's 'Halo' on fire, going 130 miles per hour through a hospital zone, being chased by helicopters and ninjas, and the ninjas are all on fire, too.

Lead Designer Jaime Griesemer pointed out that it will have multiplayer over the Internet through Microsoft's Xbox Live service, and hinted at new multiplayer game types, bringing Halo's network play more in line with the original concept of squad-based gameplay.

Halo 2 is supposed to have twice the number of pilotable vehicles.

It is not yet known whether or not Halo 2 will incorporate any of the missing or requested features, such as gameplay films, mods, multi-Xbox cooperative play or authoring tools.