Here you go, more than you might ever want to know:
Flat disk pipe this is the one that has a muffler in the rear section making them more quiet. It is basically a fixed length pipe with a muffler built into the rear end of it.
Twin cone pipe, designed to widen the power band RPM range.
Fixed length pipe this yeilds the most power from a engine, but can be very loud. It also has to be tuned as its power band can be very narrow, depending on the design.
Mini pipe fixed length not adjustable