It's called Action Replay and you can use it to get all the items, berries, money and stuff like that. You can also cheat to get hacked pokemon, alter stats, give your pokemon moves they shouldnt know. I use it to clone. I've been trying to put in a code that shows the IV's of my pokemon. And I might use it to hatch eggs faster.
I just got in! I've been gone all day. It seems like I miss a lot when I'm offline. I got a AR for the DSi and I didn't even get to put any codes in it.