Obviously I'd take issue with Chuck's comment; it's overly simplistic and comes perilously close to the reductio ad absurdam of saying that all music is folk music.  In mentioning Mozart, Praise raises an interesting point.  Composers who themselves belong firmly in the Art Music camp sometimes "borrow" traditional melodies; that doesn't make their work "folk music" in any sense, however.  On the other hand, melodies from art music do from time to time find their way into tradition; an example that comes to mind is Weber's "Huntsman's Chorus".

Malcolm