Tenebrae is a modification of the quake source that adds stencil shadows and per pixel lights to quake. Stencil shadows allow for realistic shadow effects on every object in the game world. Per pixel lighting allows you to have fine surface details correctly lit. These are essentially the same algorithms as used by the new Doom game.
It is important to understand that Tenebrae was meant as a techdemo and is (out of the box) incompatible to any mods.