ROM hacking

Rom Hacking is where you download a copy of a Pokémon game and use a rom editor (e.g. ROM editor) to change aspects of the game.

For example, the sprites, maps, trainers, can all be changed relatively easily so you can end up with a game slightly different. However, seeing as all pokemon games have simikar plots, this isnt usually a problem. Videos on YouTube tell you methods for changing the game, and this is how games such as Chaos Black, Quartz, ECT