It is a relatively simple format that allows the user to specify arbitrary name/value pairs, using UTF-8 as the character encoding.
To avoid incompatibilities between tags some standard fieldnames are suggested by the vorbis comment specification (TITLE, VERSION, ALBUM, ARTIST, TRACKNUMBER, etc.). The full vorbis comment specification can be found at http://www.xiph.org/ogg/vorbis/doc/v-comment.html.
Several unofficial efforts have been made to establish a kind of standard for vorbis comment fields specification. For more information go here http://www.gophernet.org/articles/vorbiscomment.html and here http://reference.reactor-core.org/ogg-tagging.html