ROM hacks
ROM hacking is the process of modifying a ROM image of a video game to alter the game's graphics, dialogue, levels, gameplay, and/or other elements. This is usually done by technically inclined video game fans to breathe new life into a cherished old game, as a creative outlet, or to make essentially new unofficial games using the old game's engine.
ROM hacking is generally accomplished through the use of a hex editor (a program for editing non-textual data) and various specialized tools such as tile editors, and game-specific tools which are generally used for editing levels, items, and the like, although more advanced tools such as assemblers and debuggers are occasionally used.
In the Super Mario 64 community, ROM Hacks are fangames of the original game, allowing for a new experience in a new story, with characters, abilities, or a range of other attributes defined by the game's author. Some ROM hacks are simple reskins, whereas others are entirely new games.