MP4 (formally MPEG-4 Part 14), 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 is a standard specified as a part of the MPEG-4 specifications, formally as ISO/IEC 14496-14:2003. Unlike the audio-only compression formats MP3 and MP2, MP4 is a container format that can hold various types of media from various codecs. During the 2000s, portable media players were sometimes erroneously advertised as "MP4 players", even if they may play a different format like AMV video and not necessarily the MPEG-4 Part 14 format.