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 posts that violate the SE 'Be Nice' policy
For questions about timestamps that appear in notices and tooltips upon hovering those notices.
A message displayed to alert the user to a problem or potential problem.
For questions relating to the reputation tab in user profiles.
For questions involving the "Ignored Tags" specified in a user's Preferences. Questions with these tags are dimmed or removed entirely from question lists.
Indicates a post used for submitting and voting on Community Promotion Ads.
For issues with how Stack Exchange sites look or work that are specific to Firefox.
For questions related to the support of HTTPS by Mathematics Stack Exchange, which is finally live as of spring 2017.
Score means the total number of upvotes minus the total number of downvotes.
For questions related to defining own macros (using \newcommand and similar commands) in MathJax.
For questions related to the sort orderings of questions, answers, and other lists on SE sites.
For questions and discussion related to (ab)use of the site in the context of exams.
for questions regarding unregistered users.
For questions discussing possibilities to privately contact other users.
For questions about various counters showing number of views of a particular question, profile page, ...
When viewing a question, the "Related Questions" block is shown on the right side of the page and provides a system-generated list of questions similar to the current one.
For questions about the timeline view of questions, accessible via "/posts/xxx/timeline" on any site.
Questions that have been closed as duplicates may sometimes be merged by moderators. One question is retained as a stub while all answers appear on the other.
time within which actions can be revised without a trace in the system. This includes a period in which you can undo and change your vote; a period where editing posts can be done free…
For questions about the 'being nice' behavior we expect from all Stack Exchange users. What this entails can be found in the help center on every site.
the new block that replaces the "Visit Meta" block that was before used on Stack Exchange sites.