Halo 3: ODST is an Expansion Pack for Halo 3, so it's not really an entire new game which means it will be cheaper than Halo 3. It will come out on disc though just like any other game and have 1000 Gamerscore for you to earn. It will come out in Christmas 2009. There will be an entirely new singleplayer campaign that's going to be roughly half the size of Halo 3's campaign.
You will play as an ODST (Orbital Drop Shock Trooper) and not the Master Chief, meaning you will be weaker and the gameplay will be less hectic and probably a little more stealthy. You will be dropped down onto Earth, Africa, your mission will be to find your 4 ODST friends in an open world in any order. Once you locate a soldiers position you will paly a flash-back while being that soldier to find out what happened to him. By the way, this campaign is set during the events of Halo 2.
Halo 3: ODST will also contain all the Halo 3 multiplayer map packs, as in the Heroic, Legendary and Mythic Map as well as 3 new maps which are Heretic, Citadel and Longshore. But to be able to play these maps you will need to own Halo 3 as Halo 3: ODST just an Expansion Pack and does not have it's own multiplayer.
It's going to be great!