Radeon is made by ATI, Nvidia and ATI are both graphic chipset makers, but ATI does produce its own cards and not just the graphic chipset.
ATI is a company based out of Ontario, Canada
Nvidia is based out of California.
If you need to install drivers for your card, first determine what you have,
If you have an ATI card go here: http://www.ati.com
If you have a Nvidia card go here: http://www.nvidia.com