This is a Canonical Question about Active Directory Domain Services (AD DS).
What is Active Directory? What does it do and how does it work?
How is Active Directory organized: Forest, Child Domain, Tree, Site, or OU
I find myself explaining some of what I assume is common knowledge about it almost daily. This question will, hopefully, serve as a canonical question and answer for most basic Active Directory questions. If you feel that you can improve the answer to this question, please edit away.