M4A file format is used for storage of audio data created in MPEG-4 format and encoded with ACC (Advanced Audio Coding) or ALAC (Apple Lossless Audio Codec) codec. M4A container was introduced and popularized by Apple as a solution to the problem of discerning between MP4 audio-only and MP4 audio-video files. M4A format is similar to MP4, but it stores only audio data. Both types of files use the same audio-video MPEG-4 container format.

Windows Media Player supports codecs used by M4A format since version 12. Earlier versions of the software required user to install additional codec package such as K-Lite Codecs Pack in order to play .M4A files. iTunes Store offers songs in M4A format with ACC encoding, and songs that include copyright protection have an .M4P extension. ACC compression algorithm offers both high-quality audio and reduced file size.