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 the posts about announcement of recent changes applied on site.
Comments are a small addenda to each question or answer, intentionally limited in size and formatting and intended to prompt clarifying edits.
For questions about fostering a sense of community amongst users of this site.
For questions about the Community ♦ (system) user which acts as a background process to do some system defined activities or functions.
For questions about finding data or queries with the help of The Stack Exchange Data Explorer (SEDE).