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.
keywords or labels that categorize and group your question with other, similar questions.
when examining what is on-topic or off-topic, and why (i.e scope of the site).
for questions about tag synonyms (how they work, who can create them, etc).
For questions about reputation, a measurement of a user's contributions to the site, and the extent to which the community "trusts" that user with extra privileges