I have a Cyclo DS so I can help. Here's the basics:
- It's a DS Flash Card. You put game roms and the like onto the micro SD card, slot the Cyclo into your DS/Lite (it doesn't work for DSi) and the system believes that the actual games are in there, allowing you to play them for free, essentially.
- Supports plenty of emulators for older systems such as Sega Genesis, SNES, NES, Gameboy Colour, etc.
- It's simple to use, basically just drag and drop the files onto your card, or organise them however you see fit.
- Allows you to watch movies in the right format (dpg) and play music, view images, etc through a program named Moonshell.
- It's a little more costly than your average flash card kit, but to compensate you get solid support from the development team. Example, Pokemon Heart Gold and Sould Silver still do not work on many cards, the Cyclo team released a new firmware which bypasses the game's anti piracy measures.
- Works alongside a Slot 2 expansion card called the EZ Flash 3 in 1 to allow GBA games to work. Cannot play GBA games on its own, but to my knowledge no flash card can.
The Cyclo is a great bit of kit, it's never once let me down. It's one of the best cards money can buy, I highly recommend it.
Now, if you'll excuse me, I'm gonna go break out my Cyclo and play some Soul Silver, then watch a movie on Moonshell. 'cause I can.