Domain Controller

Only domain controllers can host Active Directory. All servers that are not domain controllers must access the directory in the same manner as the workstations. They send requests for information to a domain controller, which processes the request and returns the information back to them.

Domain controllers store and maintain portions of the directory. They also have services that allow them to directly store and retrieve information from the directory. These services are referred to as the Active Directory. When you install Active Directory on a Windows 2000based server, it becomes a Windows 2000based domain controller.Reference: http://technet.microsoft.com/en-us/library/bb727049.aspx