WebM is a video codec originally developed by On2 Technologies. On May 2010, Google purchased On2 and decided to release WebM and VP8 under a royalty-free license to make it available for HTML5 video development.
Coconut's WebM video encoder will provide you with the best video quality for your project.
Note that Coconut will also soon provide full support for the brand new VP9 codec. So stay tuned.