The GBA is the Game Boy Advance, a portable system from Nintendo that succeeds the Game Boy and precedes the Nintendo DS. The GBA is a recent system; Nintendo introduced the GBA at 2001, the GBA SP at 2003 and the GBA micro at 2005. The appearance of the DS at 2004 failed to immediately displace the GBA; but the licensed hackers stopped making GBA games around 2006 or 2007. Only we, the ROM hackers not related to Nintendo, remain to do anything new with the GBA.
The GBA uses ROM from cartridges, but the GBA escapes the constraints and limitations of the older systems that also use ROM. Two common activities, when targeting the GBA, are the translation hacking and the hacking of Pokémon games. The GBA also has many ports of SNES games, but hackers tend to target the original SNES games instead of the GBA ports. (The preference by hackers for older systems, and the existing knowledge of SNES games, seem to more than compensate for the constraints and limitations of the SNES.)
To play a ROM hack, one can either pass the ROM image to a real GBA (or DS) using some unlicensed device, or load the image to an emulator. The one major emulator is the free Visual Boy Advance. The freedom of VBA allows anyone to add features; thus VBA now supports multiple operating systems and has many extra features like a memory viewer and an internal debugger.
The GBA has an ARM processor, which seems to run little-endian and mostly stay in thumb mode. The processor uses a 32-bit address space; the ROM appears as one ROM bank from address $08000000. (This is not much information; this wiki needs to add or link to more information.)
For playing the GBA:
For hacking the GBA: