The VideoLAN Knowledge Base's goal is to document codecs, file formats, protocols, etc. Although mainly consisting of technical information, it is intended to be easily understandable by all users. The Knowledge Base serves as a glossary on video/audio streaming/playing, helping users understand the rest of this wiki. When contributing to the user documentation please link to these definitions as much as possible.
In addition, Mike Melanson and Diego Biurrun, from the FFmpeg project, have started the MultimediaWiki where you can find tons of information about multimedia technology, particularly codecs.