Tags are used to classify questions into different groups. The "tags" tag should be applied to any meta question asking about tags.
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, which are shown with a red background.
For more information see the section on tagging in the FAQ.