The key difference between a performance cat back exhaust and an exhaust resonator is that the cat back exhaust system bends the exhaust output back onto itself through a Y connector pipe. This pipe is welded to the bottom of the dual exhaust system so that the power that would normally have gone straight down the exhaust pipe that is rerouted. This is once again through the exhaust system so that you are adding back pressure to the system and forcing a bit more power out of the exhaust system before it finally heads out the resonator system. The resonator system is simply an added piece of your exhaust system that adds a nice note to your exhaust.
For more info, see How Exhaust Radiators Get Rid of Rasp