An M4A file is an audio file created in the MPEG-4 format, a multimedia container format used to store compressed audio and video data. It contains audio encoded with either the Advanced Audio Coding (AAC) codec or the Apple Lossless Audio Codec (ALAC).
M4A files are similar to .MP4 video files since both file types use the MPEG-4 container format. However, M4A files only contain audio data. The use of the ".m4a" extension arose due to the confusion between MPEG-4 video files and MPEG-4 audio-only files.
The iTunes Store provides songs in the M4A format. These songs are encoded using AAC compression, which significantly reduces the size of each file. Songs that include copyright protection have an .M4P extension.
NOTE: Windows Media Player 11 requires the K-Lite Codec Pack in order to play M4A files. Windows Media Player 12 (included with Windows 7) provides native support for the M4A format.