-Stencil Buffer        

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Stencil Buffer - An on-card buffer that is used for special effects involving -Pixel Shaders. The stencil buffer allows certain flags to be set on a per-pixel basis that are then read at the last stage of rendering and used as keys for pixel shaders.

A decent explanation is at the bottom of the page here:
http://www.beyond3d.com/content/articles/41/

Stencil Buffer Tutorial:
http://developer.nvidia.com/object/Stencil_Buffer_Tutorial.html