"Active Directory is the directory service used in Windows 2000 Server and is the foundation of Windows 2000 distributed networks."
The core of Active Directory is a combination of an LDAP server and MIT Kerberos 5 KDC running on a Windows 2000 server acting as a domain controller that work as a unit to provide authentication ("Who are you?") and authorization ("What are you allowed to do?") information within a group of interlinked systems.
Above and beyond that, the LDAP "face" of this structure behaves as an enterprise-wide distributed database that not only contains Windows-specific information but can be extended to incorporate user-defined data as well.
The AD is held together by DNS, which is used not only to locate specific machines within the AD but also to locate which functions of the AD are running on which domain controllers.