NVIDIA and AMD(formerly ATi) are the manufacturers of the GPU( the graphics processing unit) and give the add-in board partners like ASUS, XFX, MSI etc. the basic design upon which they build the cards and sell them. Technically, they're all either NVIDIA cards or AMD cards. The only thing these board partners do is sometimes make custom designs, use different components etc. and sell them under specific names like MSI Twin Frozr or ASUS DirectCu. I hope you get it now.
BTW, what's your PSU?