Ive read through the manual on stencil buffers, and im curious. It seems to be a powerful tool, but it seems complex.
Im trying to decide if its worth learning it.
What would you use it for? Shadows? How would you implement it?
Thx all. 
Shadows are one application for stencil buffers. Google for “shadow volumes” for details (it’s a moderately complex algorithm).
Other applications are to make irregularly shaped viewports, or to define shiny parts of a scene to render planar reflections on.