A tag, ie a descriptive word or phrase. Entities may be tagged by users,
and later returned by a queries for that tag. Tags can also be used for
ranking results (frequency), photo captions, clustering, activity, etc.
Here's a more in-depth description: http://www.zeldman.com/daily/0405d.shtml
This is the Atom "category" element. In XML output, the tag is provided as
the term attribute. See:
http://www.atomenabled.org/developers/syndication/#category
Raises BadValueError if tag is not a string or subtype.