A Nintendo 64 ROM Patcher is a software utility that applies a set of pre-defined changes (a patch) to a ROM dump of an N64 game. Instead of distributing copyrighted game code (illegal), fan translators, modders, and hack creators distribute small patch files. The end user combines the patch with their own legal ROM copy to produce a modified, playable game.
ROM patching is the process of modifying a read-only memory image by applying a diff file (patch) that encodes changes between an original and modified version. For the Nintendo 64, patches are used for: nintendo 64 rom patcher
(which has extensive ROM hacks like Paper Mario: Hard Mode) or specialized technical documentation like the N64 Game Reverse Engineering Guide found on Scribd. Common N64 ROM Patching Tools What Is an N64 ROM Patcher