Buying Online Games

When you purchase a game through the xbox store you are purchasing the full game as if it's been bought on a disc except its a digital version there may be different versions of games available so always read the description first. As for backwards compatibility
it will run just fine. Just purchase download and play.

You would be purchasing the game and no you dont need the disc as well.

The game will be digitally downloaded and stored on your Xbox.

The game is complete and does not need disc, digital works perfectly.

As others have said once you purchase it is yours to keep (no need for a disc)

its the full game and no its not a monthly subscription its the whole game and its all set for you