SLI = Scalable Link Interface
It's really just multiple graphics cards stringed along for extra performance or dedicated graphics/physics processing.
Regular setup is..well....regular
The ATI equivalent of SLI is called CrossFire.
EDIT: Do note that SLI requires two (or three) of the exact same card (manufacturer variation is acceptable but not prefered) While CrossFire allows cards of the same "family" IE: 4870 and a 4850 - The 48xx family. The only Nvidia cards that can differ and still be SLI'd are the 8800's and 9800's because they both run on the G92 GPU.
Also, SLI and CrossFire do not exactly "double" or "triple" the performance of the card you had initially.