An acquaintance has asked this very question, as she will soon be undergoing a rather rigorous battery of certification tests. In the interests of (a) keeping my word to provide a take and (b) not writing a book, I broke this massive topic down into four main points (and post a large disclaimer regarding over-simplification at the start). Why is this topic "massive"? Consider every artist who has ever created art, multiply that by different combinations of social, personal and physical functions of art, and then multiply that by every person who ever has or will view said works of art. It might be simpler to classify grains of sand on a beach! I have infinite respect for anyone faced with this question, particularly when professional credentials are at stake. Personally, I'd be scared silly.