What is Halo 3: ODST?

If you could answer all my questions, that would be great. If I buy ODST from the store will it have 2 discs like one for the campaign and one for multiplayer or will it just be one disc with both. I heard that ODST had Halo 3's multiplayer mode, does this mean all maps (Heretic, Mystic, Legendary etc.) will already be installed to the disc? Will ODST include Halo 3's achievements?