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 built-in search functionality of this site and about searching this site using external search engines.
Questions about diamond (♦) moderators, their powers, and actions they have taken.
For questions concerning using images in questions, image hosting, etc.
For questions on usage of links at MathOverflow (both links to various parts of this site and to other sites).
no Meta Meta Mathoverflow, so questions about this very site just get this tag.
Cross posting refers to posting the same, or almost the same, question (or post) on different sites in the Stack Exchange network (or possibly also elsewhere).
Some changes to questions, such as edits or receiving new answers, will "bump" them back onto the site's front page where the changes can be vetted by other users.
Questions about suspected user spam, the spam flag or combating spam as a community
the answer the question owner thought best met their needs. Accepting is encouraged but never required. This tag is for *discussing* issues related to accepted answers, not indi…
For questions about the favorite question feature or the favorite tags feature.
Users without full edit privileges can suggest edits to posts and tag wikis
For questions about the copyrights of the Stack Exchange sites themselves or about the posting of copyrighted material by users.
For questions about the user named Community ♦ present on MathOverflow (and other Stack Exchange sites.)
should be used when you are expecting a large number of diverse answers to a question.
for problems and suggestions for the privileges gained by reputation on the main site and on meta.
For questions about Markdown, a lightweight markup language used across the entire Stack Exchange network for post formatting.