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.
Basically, considering for deletion anything that fell through the cracks when the automatic cleanup algorithms were applied.
Use for questions currently developed by the community to become a permanent part of our FAQ.
if a set of questions with wrong or insufficient tags should be characterised with new or existing tags.
For questions concerning resources to which the site is linking to.