ATI/Nvidia design the stock cards and makes the GPU itself. The actual cards are usually put together by another company who then sells them.
ATI cards are made by sapphire, xfx, HIS, ASUS, Powercolor, etc.
Nvidia cards are made by BFG, EVGA, ASUS, XFX, etc.
Does each company add different things that make their product better or worse, or is each one essentially the same thing in different packaging?