Action Replay has been around for years helping people to alter games. Yes they tend to be used for making games easier giving you things like unlimited lives, invincibility and full stock of items to name a few. Just stick the Action Replay DS into your DS a power it on. Once told remove the Action Replay and insert your game. You will then be able to choose the codes you want to use.
However the Action Replay does offer more I tend to use them for Pokemon to enable me to either capture creatures that I wouldn`t normally be able to obtain. You can also obtain new codes via the internet using the Code Management PC software supplied. The only issue with the Action Replay DS (and the Action Replay DS EZ) is that they only work on Nintendo DS and Nintendo DS Lite. If you have a DSi or DSi XL you need the Action Replay DSi which will also work on the DS and DS Lite.
The Action Replay DS (EZ) is a great device but I didn`t give it 5 stars as it`s a discontinued product and was replaced by the Action Replay DSi which will work with DS, DS Lite, DSi and DSi XL.
I would like to point out that ordering this will get you the Action Replay DS EZ which is the updated version of the Action Replay DS running the latest firmware (v 1.7.1) and comes with the Pokemon Black & White codes. Like said above the Action Replay DS was replaced with the EZ, you need to remove these to insert the game. The latest one the Action Replay DSi has you insert the game into the Action Replay to use the codes.