MPEG-4

MPEG-4 is an expansive collection of standards related to a set of multimedia formats. It is developed by the Moving Picture Experts Group, codified as ISO/IEC 14496. Saying just what the "MPEG-4 file format" is, is a somewhat complicated question. Parts 12, 14, and 15 specifically refer to "file formats".

Part 3 defines compressed audio formats, including AAC, ALS, and SLS.

Part 10: Refer to H.264.

Part 12: Refer to ISO Base Media File Format.

Part 14: Refer to MP4.

Part 15: Refer to AVC (file format).

Part 22: Refer to Open Font Format.

"MP4 audio players" don't always support the MPEG-4 file formats, sometimes supporting only the AAC audio encoding from MPEG-4, Part 3 (aka MPEG-4 Audio). These would more properly be called AAC files, with the extension ".aac".

MPEG-4 is encumbered by numerous patents, and (at least) commercial implementers have to secure licenses. Many of the licensing aspects are handled by a group called MPEG LA.

Links

 * Wikipedia: MPEG-4
 * MP4 Registration Authority
 * RFC 4337: MPEG-4 MIME type registration