I've noticed people say "the key of A minor" etc... so my understanding so far is that you can jam along the chord progression with A Minor scale. But my question is are there any restrictions in which chords you can play to make that backing, because it's in a key of something?

I know a guy who pulls out alright tunes without any theory and here I am reading posts and columns of great guitarist who talk in this language, its quite mind boggling. I try to come up with chord backings with just luck, strumming to anything I know, but its sounds like this is not the only way to go about writing solid chord progressions.

If there's a previous post regarding same topic (which I don't know the name of this theory) please post! :D

Thanks,