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MP4 is the "official" container for MPEG-4 media types defined by the Moving Picture Experts Group. It is, or will be, typically used to store data in files, though it will be used in data streams and possibly in other ways.
MP4 thus allows streaming over the internet. It also allows multiplexing of multiple video and audio streams in one file, variable frame- and bit-rates, subtitles and still images. Content other than audio and video is called "advanced content" by the MPEG-4 standard!
The formats that can be embedded in MP4 files is limited to:
- Video: MPEG-4, MPEG-2 and MPEG-1
- Audio: AAC, MP3, MP2, MP1, CELP (speech), TwinVQ (very low bitrates), SAOL (midi)
- Pictures: JPEG, PNG
- Subtitles: xmt/bt text format (means that subtitles have to be translated into xmt/bt)
File extensions MP4 uses:
- .mp4: official extension, for audio, video and advanced content (see above) files
- .m4a: for audio-only files; can safely be renamed to .mp4, though opinions differ on the wisdom of this.
- .m4p: FairPlay protected files
- .mp4v, .m4v: video-only, raw mpeg-4 video streams
- .3gp, .3g2: used by 3G mobile phones, can also store content not allowed in .mp4 (H.263, AMR, TX3G)
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