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.
Questions flagged as duplicate, and/or reasons to do so.
the amount that someone can put onto a question to entice others to answer in gaining more reputation. This is for questions around this feature.
for questions about the user profile page, which contains a list of all the questions, answers, and an audit trail of the user's activity on the site.