DOC is the equivalent of AOC in France. It denotes guaranteed quality. As with the AOC of France, this does not necessarily mean the very highest quality - simply that methods of production are controlled and you will know what you are getting.
DOCG is a higher classification than DOC, indicating the best quality in addition to controls of methods of production. Only four wines currently qualify for this award: Barolo, Barbaresco, Brunello di Montalcini and Vino Nobile di Montepulciano.