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 discussing whether specific types of questions are allowed on this site or not.
For questions about the bounty system, which allows users to spend some of their reputation in order to draw more attention to a question.
a flag that can be applied to answers (and questions) which lowers the barrier for editing and prevents any rep gain/loss from voting.
Security Stack Exchange participation in Capture-The-Flag competitions
designed to solicit opinions or best-practices on a particular topic, with the goal of reaching community consensus.