Folksonomy Tagging
Relevant video:
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.
Votes
Every editor can either upvote or downvote attaching a tag to an entity. Adding a new tag counts as upvoting. The total of votes has to be positive for the tag to be displayed.
Examples
There's no limit to what you can use tags for. For example:
Genres
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.
Nationalities
Professions
Combinations of the above
Trivia
Pleas for help
Personal opinions