cat-back exhaust is exactly what it sounds like, its from the cat (catalytic converter) to the back of the car. It is basically the piping from the cat, with the muffler and tip. If you get a cat-back exhuast, it isn't that big of a deal of you don't buy a high flow cat. The only reason for buying a high flow cat is if the stock one is clogged. high-flow cat should be purcahses when your running out of things to do.