M4A Files

The M4A format was popularized by Apple. If you come across a file with a .M4A extension, you can play it with iTunes.

The “M4” stands for MPEG-4. Essentially, these are MPEG movie files with only the audio included. Any video player than can play MPEG-4 files should be able to play M4A files also.

For more technical details, see this Wikipedia page.