The .WEBM container file format, that stores the video and audio data is based on Matroska (also know as Matroska Multimedia Container). This is an open-source format that can contain any number of multimedia file formats.
Video data stored within .WEBM files uses another open-source compression format called VP8.
.WEBM audio data is compressed using the Vorbis compression format.
The .WEBM project was initially announced by the project's sponsor, Google, in 2010. In 2011, Google announced that all videos on YouTube were being converted to the .WEBM format.