Shack Voodoo4 Feature
Basically, the T-Buffer boosts eye candy content in games. The first and foremost effect of the T-Buffer is real-time full-scene anti-aliasing. The easiest way to describe aliasing are the "jaggies" or "stair casing" that one can see on the edges of polygons. Usually these are the most apparent at lower resolutions (and/or larger displays). Increasing the resolution reduces the "artifact" somewhat, but aliasing is still noticeable even at high resolutions like 1600x1200.
You can also check out the Voodoomag, VoodooExtreme, CombatSim, and GameSpot articles.