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