You can add folksonomy tags to any entity in MusicBrainz except URLs. They are comma-separated, so they may consist of several words. You can use any Unicode character (except for commas) as part of your tags, but any letters will be lowercased automatically.
There's no limit to what you can use tags for. For example:
Keep in mind not every genre is yet in the allowed genre list - but you can still use any as tags, and propose the genre for inclusion. See Genre for more info.
Combinations of the above
Pleas for help