Straight from Apple:<br>AAC: MPEG-4 audio<br> AAC (for Advanced Audio Coding, a big part of the MPEG-4 specification) is the cutting-edge audio codec thatís perfect for the Internet. AAC encoding compresses much more efficiently than older formats like MP3 (which iTunes still supports, by the way), while delivering quality rivaling that of uncompressed CD audio. In fact, expert listeners have judged AAC audio files compressed at 128 kbps (stereo) to be virtually indistinguishable from the original uncompressed audio source. iTunes 4 and QuickTime 6.2 is all you need to get started.<br><br> <br><br><br>My stuff for sale.