QuickTime 6 was an early adopter of MPEG-4 technology, and they seem to be big believers in the standard. This is in contrast to Microsoft’s Windows Media 9, which uses proprietary codecs to compete with the open standards of MPEG-4. The MPEG-4 standard uses AAC for encoding audio, and Apple has a site
with a few links and information about this audio encoder. Alas, they
don’t have a date when AAC will be included in the iPod!

http://www.apple.com/mpeg4/aac/