The MPEG-4 Part 14 or MP4 format, which is marked by the extension MP4 as avi and mov a container format. It is based on Apple Quicktime.
These files can contain video and / or multiple audio streams as well as subtitle text and images. It also supports multimedia content such as 2D and 3D data, menus or interactive content. Theoretically, almost any type of data can be included in an mp4, but this is not widely supported.
The video and data streams can be coded in different compression methods.
Supported MPEG 1, 2 and 4. MPEG 4 contains such codecs as Xvid, Divx and H264 and is mostly used. Audio can be in AAC, MP3, MP2 or AC3. There are other special codecs supported. Most commonly AAC is used.
MP4 files can also contain additional metadata.
The contents of the file may also be protected by DRM (Digital Rights Management). Then you need a corresponding, possibly paid license to view the content.
For MP4 files, which contain only one type of data, other endings come into use. m4v for video, m4a for audio, m4b for audiobooks.
Often, portable hardware players are referred to as MP4 players if they can play videos in addition to audio files. However, this does not mean that they can handle MP4 files.
Most players can handle MP4 files as long as they can decode the data they contain. That's the case with most videos.