MPEG-4
From Just Solve the File Format Problem
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
