From Hydrogenaudio Knowledgebase
It is a relatively simple format that allows the user to specify arbitrary fieldname/value pairs, using UTF-8 Unicode 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.). Every fieldname is allowed and there is no obligatory format for values. One Fieldnames can be used several times. In contrast to ID3 it is a it's quite free of regulations.
Several unofficial efforts have been made as proposals and recommendations for Vorbis Comment specification. These documents are available on Web, first one is called Ogg Vorbis Comment Field Recommendations and second one is called:Proposals for extending Ogg Vorbis comments.