[bena]

Sort of. ROMHacks are modified ROM images of a certain game.

If you knew where in the ROM image the level data was contained, you could modify it. As long as you didn't violate any constraints, the game would run fine.

You could also potentially influence game behavior as well.

The Game Genie and Gameshark were kind based on this concept. Except, being further along the chain, it could write values coming into and out of memory, so other effects were possible.

So, in the case of Super Mario Bros. ROMHacks, they all use Super Mario Bros. as a base ROM. Then from there, all you need to do is store the diff from the base.