SLI is basically using two identicle cards to improve performance. If you have an SLI compatable motherboard, or are considering upgrading soon...you may want to look more into it. Try a google search, there's alot about SLI and its applications.
My buddy has SLI, and he explains it like this, it is two video cards, and they lead to one monitor, each card does every other line on your computer screen which would obviously lead to a much clearer picture.