Unlike simple video files, MP4 is a "multimedia container." It can store video, audio, subtitles, and still images all in one file.
The format supports "progressive downloading," allowing users to start watching a video while it is still being downloaded from a server. Common Distinctions
You may see files with extensions like .m4a (audio only) or .m4v (often used by Apple for video with DRM protection), both of which are based on the same Part 14 standard.