From Mac Guides
AAC (Advanced Audio Coding) is a lossy compression audio codec which has become popularized by Apple QuickTime. Its extensions are .aac, .mp4, and .m4a. It also has a protected format, under Apple's FairPlay digital rights management (DRM) with the .m4p (p = protected) extension. .m4a is the extension imported songs receive from iTunes.
The iTunes Music Store (found in iTunes 4 and above) features protected AAC files (.m4p) which are available for download at US$.99 a song for 128 kbps bitrate. This roughly equals the quality of a 160 kbps MP3 file. Because it is the same file size as a 128 kbps MP3, but sounds better than a 128 kbps MP3, it has become quite popular. It is now supported in RealPlayer, and as an add-in for Windows Media Player.