• Active Directory expands on the domain concept by linking domains in logical structures named trees, and multiple trees into forests.
• Domain controllers are servers that hold a copy of Active Directory information and are responsible for authenticating users when they log on to a workstation. Domain controllers also respond to requests for other domain information such as printer information or application configuration.

Learn more about Active Directory at:
http://learnthat.com/2008/07/introduction-to-active-directory.