I'm not a specialist, but I have rebuilt plenty of transmissions, and I'll at least be courteous enough to take the time to try to answer your question, since no one else has.
As you remove the transmission oil pan, the main internal part that you see, is the valve body. It's simply the case that retains the valves, that control the fluid pressure for each gear shift.
The reason that it's called an overhaul, is that most everything that is a wearing component, will most likely be replaced, such as clutch discs, steel discs, o-rings, seals, etc., but not usually the hard parts, such as clutch drums/pistons, gear sets, shafts, etc., unless they are excessively worn or damaged, and need to be replaced.
It's possible that you could even rebuild the auto trans, yourself, providing that you have the tools that you'll need, and at least a reference manual to guide you through it.
If you're referring to your Camaro tranny, the tools will cost around twice what you can have the transmission rebuilt for, at your local shop. I know, because I bought all the tools, to not pay someone else to do what I'm capable of doing, myself!
Hope that gives you an idea, anyway.