Gameboy
Flash Advance Cards (also 4 GBA SP)
Gameboy
roms for GB games are what MP3 files are for Music. Flash
Advance Cards are Just like a ReWritable CDs, Memory Stick from
Sony, or like a Smart Card or Smart Media Card you are using for
your MP3 Player, it will simply hold the game data in it's Flash
Memory. Once you have sent games, demos or programs to it's memory
using the Flash Advance Linker, it can simply be plugged into
the Gameboy Advance and it will act like an original game - there
is 100 percent not any difference!
Flash Advance Card will also allow you to store savegame
data. This means once you've opened a lot of levels or you got
some extra points in your game, you won't need to start from the
scratch - it will save the game status just like an original cartridge
with the game. Savegame data can be stored on your local PC Harddrive
too, so you won't loose any data when you are using the flash
card for other games.
Flash Advance Card can be re-written, feel free to use
as often as you like, just the same like any other Flash Card
on the market (Smart Media, Memory Stick etc.).
The Flash Advance Card is furthermore the perfect choice for
any professional Gameboy Advance developersor even home developers
- Create your own games using your PC (free software is available)
and simply use the Flash Advance Card to show your game/demo to
friendsor test it on real Hardware. If you believe it or not,
even large software companies are developing their games using
exactly this Hardware and this methods.
Also if you own one of the old Gameboy COLOR units or GB B/W
you can use GBA Flash Card plugged into GB-Bridge
adapter to play GB and GBC roms! You also need GBBridge
to play gbc roms on Game Boy Advance.
One of the coolest features of a Flash Card is that you can run
3rd party software on it - that includes emulators of other Video
Game systems > NES
roms SEGA roms Z80 and more...
Update: All GBA Flash Cards
are GBA SP compatible! Gameboy
Advance SP inside is the same as GB Advance - only the shape
of the new SP is different and the screen is backlit! details
Just like you need a CD Recorder to write MP3 to CD or an adapter
to write files to Memory Stick you will have to use
Flash Advance Linker - adapter that lets you write and read
Gameboy cartridges!
GBA
Flash Linker 
New Xtreme
cards
The
things you can do with GBA Flash Card 
- SuperCard DSTWO 3DS - for playing GBA ROMs on DSi and 3DS.
- R4 3DS - the official R4i Gold SDHC Nintendo flash cartridge adapter website.
- R4 3DS Card - reviews, firmware updates, comparison of all rom backup tools.
- AceKard 3 - cheap 3DS compatible alternative cart with good support.
- R4 3DS Mod-Chip - How to Jailbreak and Unlock N3DS for playing copied games.
- R4 DS ROMs - Nintendo DS Roms, 3DS ROM files and emulators for classic consoles.
- 3DS Emulator - PC Windows 7 & Vista, Android and for Mac OSX.