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 English Language Learners, which is another site in SE network concerned with English language, but with a somewhat different scope.
is used for questions specific to the chat rooms associated with EL&U, which can be found on chat.stackexchange.com.
For questions about the bounty system, which allows users to spend some of their reputation in order to draw more attention to a question.
Questions about how to promote English Language & Usage SE, in order to attract more users.
Questions about the titles of questions on the main site.