Nintendo Game Boy
The Nintendo Game Boy was a handheld game console produced by Nintendo. The Game Boy uses a custom Sharp Z80 processor. The Game Boy utilized ROM cartridges for gameplay.
Game Boy ROM images usually have the extension .GB.
More non-ROM hacking specific information can be found at Wikipedia's Game Boy article.
The Nintendo Game Boy system was released in 1988 after Nintendo's successful Game & Watch systems. These differed in the fact that the Game Boy system had completely interchangable games, whereas the older Game & Watch systems had but one for each console. This immediately brought on fandom to the Game Boy, despite the Sega Game Gear, already on the market, being in colour. The Nintendo range of games included the ever popular Mario Brothers, Pokemon, Zelda and Tetris and boasted a new feature; the link port. This port could potentially link up to four Game Boy systems together, encouraging fighting and trading between games. However, this function was never taken beyond the standard two Game Boy link.
In 1998, ten years after the original Game Boy release, and two years after the Game Boy pocket (a smaller, AAA batteried version of the now-bulky Game Boy) Nintendo released a new version, the Game Boy Colour. Due to this new version, and having no competition throughout the entirity of its lifetime, the Game Boy Colour sold extremely well, helped by colour (DX) versions of old favorites and new. The Game Boy Colour itself was backwards compatible with all older model Game Boy games, a trend that would continue for some time albiet not quite as well, and such was a reason to completely upgrade from the older versions.