Thanks a bunch!  I'll read the links posted to try figure out a bit more, but as 
@Grunk said, this at least gets me started. 
I'm sure there are a ton of tiny things that are not compatible from one type to the next, and that's where the really hard part is - knowing which parts work in which model.
As far as AR types go, it's pretty much plug-n-play with the 5.56 since they all are based off 'Mil-Spec', and as long as you're not trying to do something 
relatively complicated like adding a piston system or changing calibers, a simple AR is easy to build.  I'd never owned one when I built my first using parts I got on the interwebz and a YT vid for instructions.  Still have it, I've upgraded it a few times (better barrel, trigger, scope) and it works great.  Grunk has the second one I built, and it's never given either of us any trouble.  High performance ARs can be a bit more work, same for different calibers and can-ready folding pistols like mine, but it's just a bit of research.  The .308AR is another story. No 'Mil-spec' to rely on, and many different platforms that aren't exactly compatible, and many parts sellers out there that don't understand the differences.  Building one from an 80% is yet another level of challenge, but I enjoyed it.
Feel free to ask me anything about it, I'll tell ya' if I know, and won't BS you if I don't.