Tags are keywords or labels that categorize and group your question with other, similar questions.
Every question asked on Stack Exchange can have up to five tags that categorize the question.
Meta sites have tags that need to be present in the question, which are bug, support, discussion and feature-request; there are also tags that can be added only from moderators, such as faq, featured, and status-declined (along with other status tags), which are shown with a red background.
For more information see the section on tagging in the FAQ.
