This little marvel enables you to extract the .sav file (saved data) from any Nintendo DS cart and save it on your computer (has to be pc windows). It is basically a USB stick with a slot for the NDS cart. The interface is quite straightforward as you can either save (from the cart to the pc) or load (from the pc to the cart). The "Plus" means that it can handle newer NDS carts when compared to the plain old NDS Adaptor.
It's made in Hong-Kong by a company called EMS. Here's more info about it straight from the maker:
NDS Adaptor Plus
You can order it from here (I am sure there are other sources):
NDS Adaptor Plus at ConsoleSource
From what I've heard, it's quite reliable.