How do you rate Guitar players? – Instructional books, videos and DVD's refer to three levels of Beginner/Intermediate and Advanced, but how do you define each category?

The author of one example of Tab I looked at recently stated that you needed to be an experienced/advanced player to play the song, but I'd rate it at no more than Intermediate. I assume there are properly defined grades, and I'd be grateful if someone could point me in the right direction, or tell me what those definitions are please.

(I'm considering some guitar teaching, so I probably need to know!)