.m4a

.m4a is the file extension attached to names of files containing MPEG-4 Audio. Generally, MPEG-4 files are have the .mp4 file extension.

The .m4a file extension has been popularized by Apple, which started using the .m4a file extension in their iTunes software and iPod music players to distinguish between MPEG-4 Video and Audio files. Currently, most software that supports MPEG-4 Audio also supports the .m4a extension. The most common type of .m4a files available are those using the AAC (Advanced Audio Coding) audio format, but other formats such as Apple Lossless and even mp3 files may be put inside a .m4a container file. You can normally safely rename the file extension of an .mp4 file containing only audio to .m4a or vice versa to get the file to properly play in your favorite audio player.

Protected MPEG-4 Audio

When these files have digital rights management (DRM) restrictions applied to them, their extension are often changed to .m4p, the p standing for protected. Music files purchased from the iTunes Music Store, for example, have this extension.

Audiobooks

When they contain audiobook data, MPEG-4 files have a .m4b extension.