What is AAC?
Advanced Audio Coding, or AAC, is a MPEG (Motion Pictures Experts Group) audio standard first adopted as part of the MPEG-2 family of standards. Like its predecessor, MP3, AAC is a Lossy Compression format capable of delivering relatively high quality at relatively low bitrates. There are actually two AAC specifications. In addition to the MPEG-2 version of AAC, there’s a newer specification developed for MPEG-4. This version is normally found in the MP4 Container, either with or without accompanying video.
What is M4A?
Audio-only MPEG-4 files generally have a .m4a extension. This is especially true of non-protected content.
What is M4B?
Audio book and podcast files, which also contain metadata including chapter markers, images, and hyperlinks, can use the extension .m4a, but more commonly use the .m4b extension. An .m4a audio file cannot “bookmark” (remember the last listening spot), whereas .m4b extension files can.
What is M4P?
MPEG-4 files with audio streams encrypted by FairPlay Digital Rights Management as sold through the iTunes Store use the .m4p extension. iTunes Plus tracks are unencrypted and use .m4a accordingly.
What is M4R?
The Apple iPhone uses MPEG-4 audio for its ringtones but uses the .m4r extension rather than the .m4a extension.