SLI is using two nvidia graphics cards in the same computer. It provides more performance in games. It also requires a motherboard with at least two 16x PCI-E slots, and a lot of power. And i think your cpu must support it as well, but i'm not certain.
CrossFire is Ati's equivalent of SLI. two graphics cards, one computer, more performance.
See the SLI 101 sticky in video cards/monitors section.