Advanced Audio Coding
AAC is a sound coding standardized by ISO. Compared to MP3, the AAC provides superior music reproduction quality for lower bit rates and can accommodate up to 48 sound channels.
This format has been designed among others for the purpose of streaming. As a result, it is the format that was chosen to encode the music of the iTunes Music Store, rather than the MP3 format for example. Despite its intrinsic qualities, it is still far from equaling the omnipresence of MP3.