I won't go into the 101 too deep as you guys know just fine what SLI and Crossfire is these days. But basically you combine multiple GPU's, and have them work together to render games faster or with higher image quality settings.

This is something you really need to keep in mind though: Multi gaming is intended for people that game at high-resolutions, preferable 2560x1600. But with more complex games the gain is quite visible in say 1920x1200 as well. Everything below that is likely a waste of the investment.

The first installment of Quad-SLI flopped. Two 7950 GX2 cards setup with Windows XP just did not work well. Apparently this limitation was more a DirectX9 issue rather than game engine compatibility. DX9 restricts the number of back buffers that can be queued. Very blunt explanation: DirectX will not allow 4-way AFR which is what we really need for it to make Multi-GPU rendering happen.

AFR - Alternate Frame Rendering. Each graphics core is rendering a frame at a time, odd and even. The full frame is prepared by the CPU and forwarded to the driver, which makes it render on each device to render a frame in turn. This is interleaving. First card A does a frame then card B the other.

Both NVIDIA and ATI use very similar rendering methods. The oldest generation multi-GPU rendering (Quad SLI approach 7950 GX2) was a mixture of split frame rendering (SFR) and alternate frame rendering (AFR) to enable parallel rendering on four GPUs.

With the latest games utilizing complex shaders, inter-frame effects, and multi-pass rendering, SFR became less efficient. A new video bridge and a re-architected drivers will these days allow for four frames to be rendered parallel. With 4-way AFR, geometry, texture, and shader processing.

With the arrival of Microsoft Windows Vista the original problem is gone, opening the way for NVIDIA and ATI to continue to develop multi-GPU gaming.