Disc 2 of Halo 3: ODST is the Halo 3 multiplayer experience.
This means that it contains everything on your current Halo 3 disc apart from the campaign.
All maps will be on the disc (original, heroic, legendary, cold storage, mythic, mythic 2).
Completing the 4 vidmaster achievemnts on Halo 3 and the other 3 on Halo 3: ODST will unlock you recon armour for Halo 3 multiplayer.
Disc 1 contains Halo 3: ODST campaign and firefight modes. The character 'Dare' (The one wearing a recon helmet) will be unlockable for the firefight mode to play as.