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.
For questions about the bounty system, which allows users to spend some of their reputation in order to draw more attention to a question.
For questions about the Winter Bash (or holiday hats) to celebrate the end of the year.
for questions specifically about the process of answering questions on the Graphic Design Stack Exchange site.
Questions related to customs and rules for polite behaviour.
Noting something well done