What is an MP4?
Just like MP3s work by compressing music and audio files without affecting the audio quality, MP4 or MPEG-4 (ISO 14496) files are compressed video files. MP4 is also known as MPEG-4 AVC, or advanced video coding. This new technology is going to make video files decrease in size so that they are easier to work with and support. What used to be massive video files of motion and audio are now compressed versions of the same material, called MP4s. MP4 is considered a container format which allows you to combine different multimedia streams (most of the time audio and video) into one single file. Multimedia containers are for example the well known AVI (.avi), MPEG (.mpg, .mpeg), Matroska (.mkv, .mka), OGM (.ogm), Quicktime (.mov) or Realmedia (.rm, .rmvb) MP4 is the global file extension for the official container format defined in the MPEG-4 standard (ISO 14496-14)

MP4 Video Players
The most famous MP4 player are the Apple iPods, that is the Video iPod and the new iPod Touch, basically the video playback models. Other players are the Sony PSP, Microsoft Zune, and 3GP cell phones. Another way to look at it is, if it can fit in your hand and it plays video, it more than likely is an MP4 player.

MP4 Codec
MP4s produce videos of DVD-quality at less than 1 Mbps through a broadband connection. This means that with a MP4 player, you can retrieve videos through an Internet connection and watch them with your MP4 player. MP4 players can also play lower MP versions, such as standard audio MP3s.

MP4 Extensions
- .mp4: is the only official extension, for both audio and video files (and advanced content)
- .m4v: .mp4 files with the WRONG extension introduced by Apple for video+audio files, .m4v can be safely renamed to .mp4 (.m4v is used in Apple iTunes and iPod)
- .m4a: introduced by apple for aac/alac audio-only files, m4a can safely be renamed to .mp4
- .m4p: DRM protected files sold in iTunes, using the DRM sheme developed by apple
- .m4e: renamed .sdp files used by Envivio for streaming
- .m4v, .mp4v, .cmp, .divx, .xvid: video-only, raw mpeg-4 video streams (not inside MP4)
- .3gp, .3g2: used by mobile phones, stores content not defined in .mp4 too (H.263, AMR)