Cat-back system is an entire system that replaces everything after the catalytic converters. It includes the mufflers/resonators and tailpipes.
Axleback is just everything behind the axles. I think it only applies to the '05 and up models because their mufflers are after the axle...I think...
The midpipe is either an x-pipe, h-pipe, or prochamber. Also you can just go straight pipes without any type of crossover. An x-pipe will give a few more horses but will sound raspy. An h-pipe will give you a classic muscle car rumble sound. It also depends on which mufflers you choose.
Headers are either longtube or shorties. Longtubes are hard to install but will give a better horsepower gain than the shorties.