MPEG-4 Part 14, or MP4, is a digital multimedia container format most commonly used to store video and audio, but it can also be used to store other data such as subtitles and still images. Like most modern container formats, it allows streaming over the Internet. The only filename extension for MPEG-4 Part 14 files as defined by the specification is .mp4. MPEG-4 Part 14 (formally ISO/IEC 14496-14:2003) is a standard specified as a part of MPEG-4.
Portable media players are sometimes advertised as "MP4 players", although some are simply MP3 players that also play AMV video or some other video format and do not necessarily play the MPEG-4 Part 14 format.