What is action reply?

Action Replay is like a GameShark for the Nintendo DS or in other Words a cheat device. Action Replay can be used to basically put cheats in the game such as invincibility, infinity life or in the case of Pokemon games it can also give you infinity Master Balls as well as a complete Poke Dex additionally cheats that gamers could use would include ones such as ultimate weapons never running out of ammo or having infinity money.