Tags are keywords or labels that categorize and group your question with other, similar questions.

Every question asked on a Stack Exchange site can have up to five tags that categorize the question.

Meta sites have tags that need to be present in the question, which are , , and ; there are also tags that can be added only from moderators, such as , , and , which are shown with a red background.

For more information see the section on tagging in the Stack Overflow FAQ.

Note: Content copied from http://meta.stackoverflow.com/tags/tags/info.