We don't know much so far, but what we do know is that HALO 2 has been confirmed for a 2003 release. The game will put you (and your closest friend) back into the role of Master Chief, and gameplay is supposed to be along the same lines as HALO: Combat Evolved. That is, along the same lines, but much, much bigger. Battles should be huge, and the arsenal at your disposal is said to be much larger.
HALO 2 will definitely support online multiplayer via the Xbox LIVE network (set to launch November 15, 2002). In addition to the standard modes of HALO multiplayer we've grown to love, there will be some new modes that take advantage of a much larger online multiplayer environment. Imagine battles like we saw in HALO except where every marine, alien, and Master Chief is controlled by a real, live person.
Expect squad-based tactics to take center stage again, and if these screens are any indication, it looks like we'll be treated to another milestone in graphical quality. We'll update this preview as more information becomes available.