Cats = Catalytic converter .
Cat Back = replacing the exhaust system from the Catalytic converter to back of the car.
Catted "X" Pipe = stock our cars comes with a "H" pipe" with Catalytic Converter but you can replace it by a "H" pipe off road ( without catalytic converter ) or for better exhaust flow you can use a "X" pipe with cats or Off road ( No cats ) .