AAC - Stands for Advanced Audio Coding. It's a type of digital audio file and has been promoted as the successor to the once-dominant digital audio filetype, MP3. AAC generally offers higher-quality sound than an MP3 while using the same amount of disk space.
While AAC is often perceived as an proprietary Apple format, it was in fact developed by a group of companies including AT&T Bell Labs, Dolby, Sony, and Nokia. As a result of this, AAC files can be played on a range of non-Apple devices, including the Microsoft Zune, Sony PlayStation 3 and PSP, the Nintendo Wii, and mobile phones running Google's Android OS, among many others.
How AAC Works
Like MP3, AAC is a lossy format, meaning that in order to compress CD-quality audio files into smaller files suitable for transmission over the Internet, some data that will not impact the listening experience of the file is removed. As a result of the compression, though, AAC files do not sound identical to CD-quality files.
Like MP3s, the quality of an AAC file is measured based on its bit rate, rendered as kbps. Also like MP3, common bit rates for AAC files include 128 kbps, 192 kbps, and 256 kbps.
The reasons that AAC files produce better sounding files at the same size than MP3s are very complex. To learn more of the details of this, read the Wikipedia article on AAC.
iTunes & AAC
AAC is the the format of all songs sold at the iTunes Store. With the introduction of iTunes Plus in May 2007, all AAC files sold at iTunes are 256 kbps.
AAC files purchased from iTunes can only be authorized to play on a maximum of 5 computers. Computers can be authorized and deauthorized regularly, depending on your needs.
iPod/iPhone/iPad compatible: Yes