I'm planning on building an entirely new rig for myself, and will have some pieces left over from my old one when it's all said and done. I was considering building an HTPC with the pieces that were left, but then I realized that I didn't really know anything about that one way or the other.

So, I guess my question is what is are the uses of a HTPC, and how does one go about building one? I know it sounds rudimentary, but I really don't know.