Videos are stored as single file with a specific file extension in so-called containers. This container contains one or more video and audio tracks, each one encoded with a specific codec. A codec is essentially a compression algorithm as well as a description how data is filed. Unfortunately, some file extension are also used for container as well as codec names. Because of this, you can never solely rely on a video label, as it could stand for either the codec, the container or the file extension.