Video is almost always stored using lossy compression to reduce the file size.
A standardized (or in some cases de facto standard) video file type such as .webm is a profile specified by a restriction on which container format and which video and audio compression formats are allowed.
In contrast to that, some very general-purpose container types like AVI (.avi) and QuickTime (.mov) can contain video and audio in almost any format, and have file extensions named after the container type, making it very hard for the end user to use the file extension to derive which codec or program to use to play the files.
The free software FFmpeg project's libraries have very wide support for encoding and decoding video file formats.
For example, Google uses ffmpeg to support a wide range of upload video formats for YouTube.