In short they both make Graphic Cards, Ati calls them (at the moment) Radeon and Nvidia calls them (at the moment) Gforce in tota Nvidia has a dual card capability that works thru the MB names SLI, ATI has a dual card Capability that works independent from the MB and connects them together with crosslink wires hence the name "Crossfire",
Having said all that I can nearly guarantee that the SLI solution from Nvidia works wayyy better for games <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
