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 to establish a kind of standard for Vorbis Comment specification. More information here