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.
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.
There's no limit to what you can use tags for. For example:
Combinations of the above
Pleas for help