What is MP4? I have heard it is a "container format"? What's that?
 

A container format 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
MP4 is streamable and supports all kinds of multimedia content (multiple audio-, video-, subtitlestreams, pictures, variable-framerates, -bitrates, -samplerates...) and advanced content (officially called "Rich Media" or "BIFS") like 2D and 3D animated graphics, user interactivity, DVD-like menus... - next to nothing handled by the often used AVI
I see it on one page, it may be give some help..