A tag is a keyword or label that categorizes your question with other, similar questions. Using the right tags makes it easier for others to find and answer your question.
Users who post without an account.
When the question deals with the format of the answers posted on Ask Ubuntu.
For questions about the bounty system, which allows users to spend some of their reputation in order to draw more attention to a question.
to be used when requesting a deletion (but not blacklisting) of a tag on either the main or meta Ask Ubuntu site.
about the Ask Ubuntu Clean Up meant to clean up the site
a popup that lists available close reasons in order to vote to close a question. This tag is only for questions about the dialog itself.
Questions which have been closed for any of the usual reasons.