The .AAC (Advanced Audio Coding) file format is most commonly known as the default audio format for Apple's iPhone and iPod devices, as well as being the format used by the iTunes store. It's also the standard format for Sony's Playstation 3.

The .ACC format supports extensions for proprietary digital rights management (DRM) meaning you'll need an ACC player that supports these extensions if you wish to play these files in anything other iTunes.

Compared to the MP3 format, the AAC audio format produces better quality audio at the same bit-rate.