I know what the mid-pipe is but what in gods name is the definition of and s-pipe and a down-pipe?

 

All 3 of those terms are basically interchangeable. The mid-pipe or s-pipe connects the header/exhaust manifold to the exhaust system. Ususally down pipe refers to turbo cars as the pipe coming out of the turbo to the exhuast system, but some may use it to describe the s-pipe/mid-pipe. Hope that helps.