A WEBM file is a compressed video saved in the WebM format, an open, high-quality video standard. It stores video compressed using VP8 technology and audio compressed using Ogg Vorbis compression. WEBM files are commonly used for delivering online videos using the HTML5 <video> tag.
The WEBM container format is based on the Matroska container, which stores Matroska video in .MKV files. Most major Web browsers will support WebM as part of HTML5 video delivery. WebM is an alternative to the patented h.264 and MPEG4 standards, and is suitable for commercial and non-commercial applications.
NOTE: Google acquired ON2, the original developer of VP8 technology. VP8 is now an open and free technology.