Correct. S-Off allows you to flash anything to the phone regardless of whether it belongs on there or not. Caution should be exercised when you have S-Off. One use for a RUU was a means of flashing the phone back to stock with software leaked from HTC. We don't see much of them anymore.
There are, of course, many other things that both of these things can do. Look in the M8 International forum (http://forum.xda-developers.com/htc-one-m8
) for a much more in depth explanation of both. They have step by step instructions for almost anything you'd need. The ROM's over there are usually applicable to the AT&T model. If not, they'll state it in the thread.
Read, Read, Read. And take it slowly. You could easily brick your phone and void the warranty as well with some of these things.