What is a CAT-BACK exhaust system?
that,s the intermediate pipe that goes from the back of the cat to the muffler.
It means you're replacing the exhaust from the catalytic converter back, meaning you're replacing all but the converter itself.
That's a term used when you replace the exhaust system from the catalytic converter all the way back to the tailpipe, but not the cat itself. This would include the muffler and tailpipe, contrary to one of the answers below.
