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 concerning using images in questions, image hosting, etc.
For questions concerning citation of posts from MO and citation of other resources when posting here.
For questions about the review queues.
Questions about diamond (♦) moderators, their powers, and actions they have taken.
For questions concerning tag-synonyms.
for questions and bug reports related to the SE chat system.
For questions about built-in search functionality of this site and about searching this site using external search engines.
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).
For questions about Markdown, a lightweight markup language used across the entire Stack Exchange network for post formatting.
no Meta Meta Mathoverflow, so questions about this very site just get this tag.
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.
For questions or issues surrounding login to this site.
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 copyrights of the Stack Exchange sites themselves or about the posting of copyrighted material by users.
For issues regarding the help center of MathOverflow.
For questions about comment @replies
For questions concerning area51 proposals.
Users without full edit privileges can suggest edits to posts and tag wikis
For questions which focus on a particular user, rather than those discussing general user issues.
for problems and suggestions for the privileges gained by reputation on the main site and on meta.
For questions about the user named Community ♦ present on MathOverflow (and other Stack Exchange sites.)