Can someone define exhausts for me

A "cat back" exhaust system means everything from the cats backwards to the exit at the rear of the car.

Normally, there is a flange just behind the cats. This makes an easy point to split the exhaust system. The after market exhaust replaces everything from the flange backwards. This leaves the factory cats in place. By doing it this way, if avoids the expense of new cats as well as other issues that replacing the cats would entail.