You may think of video files as “AVI files” or “MP4 files.” In reality, “AVI” and “MP4″ are just container formats. Just like a ZIP file can contain any sort of file within it, video container formats only define how to store things within them, not what kinds of data are stored. (It’s a little more complicated than that, because not all video streams are compatible with all container formats, but never mind that for now.) A video file usually contains multiple tracks — a video track (without audio), one or more audio tracks (without video), one or more subtitle/caption tracks, and so forth. Tracks are usually interrelated; an audio track contains markers within it to help synchronize the audio with the video, and a subtitle track contains time codes marking when each phrase should be displayed. Individual tracks can have metadata, such as the aspect ratio of a video track, or the language of an audio or subtitle track. Containers can also have metadata, such as the title of the video itself, cover art for the video, episode numbers (for television shows), and so on. Read more – ‘Mark Pilgrim – A Gentle Introduction to Video Encoding: Container Formats’.