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 user profile page, which contains a list of all the questions, answers, and an audit trail of the user's activity on the site.
for questions or issues surrounding login to this site.
"Design" refers to front-end elements, such as page layout, coloring conventions, etc.
for questions specifically about the process of asking questions on a Stack Exchange site.
for questions about the various user interface and email elements used to notify users of events on the site.