Disk 2 contains (IIRC):
- Halo3 multiplayer (you won't need your halo3 disk anymore for mp :D which is handy if you've lost or damaged it )
- Every single halo3 map ever (including 3 new ones)
- Forge
i dont know if forge is right and if there's anything else..
[Edited on 06.17.2009 1:54 PM PDT]