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.
Use this tag for questions about existing tags. For discussions on what a meta-tag is or if an existing / proposed tag is meta - consider using meta - especially on meta - we might love disgussing if meta-tag on meta is indeed a meta-meta-tag.
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, which are shown with a red background.
For more information see the section on tagging in the FAQ.
This tag wiki is from the Meta.SE tags.