WebM is a container format for audio and video data, providing open video compression for HTML5 videos. A WebM Project, sponsored by Google Inc., was put together to support and release WebM content under a BSD license, which makes it available to users for free.
Additional WebM information
WebM was originally released on May 19, 2010, with support for Vorbis audio and VP8 video. Several browsers, including Chrome, Firefox, and Opera, provide native support for WebM. Internet Explorer can support WebM as well, but requires third party WebM software to do so.
The Safari browser does not support WebM because it uses QuickTime for audio and video playback, which does not support WebM. However, a third-party plug-in can be installed to provide WebM support in Safari.