A Flash Advance cartridge is a Gameboy Advance cartridge with special flash memory which allows it to be programmed. To program it you need a device named a Flash Linker. In general people use these special cartridges to play pirate games downloaded from the internet though some people use them as an alternative to Nintendo's expensive dev kit to create their own games and demos. Another useful feature is the ability to read and write savegames on any GBA cartridge of course the saved games can then be hacked about with. With new flash cartridges it is possible to fit many games onto one cart though the Flash Advance cartridges are more expensive than a normal cartridge (~150€), they can of course be reused many many times.