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.
Questions about best practice questions at issue on the main site with people asking about them or why they don't work.
Indicates you've found an erroneous or unexpected behaviour in the system that needs to be fixed.
Meta questions about why particular questions on the main site were closed.
Community Wiki is a flag that can be applied to answers (and questions) which lowers the barrier for editing and prevents any rep gain/loss from voting.
Copyright - don't breach it