SLI is an acronym or abbreviation for Scan-Line Interleave or Scalable Link Interface. It is a technology that allows multiple video cards to contribute to the rendering of complex three-dimensional graphics. SLI is often used to increase the performance of high-end video games.
Scan-Line Interleave is the old 3DFx version of the technology. The scene is rendered line-by-line, and every other line is rendered by the second card.
Scalable Link Interface is NVidia's version. It balances the load across the two cards better.
Alternate Frame Rendering or AFR is ATI's version of the technology.
A device known as an SLI bridge can be used to connect the two cards. In this configuration, the two cards communicate directly, rather than over the bus.