ActiveDirectory is MicrosoftCorporation's trademarked directory service, an integral part of the Windows 2000 architecture. Like other directory services, such as Novell Directory Services (NDS), Active Directory is a centralised system that automates network management of user data, security, and distributed resources, and enables interoperation with other directories - so long as these directories manage to conform to MicrosoftCorporation's odd and unpublished "standards". Active Directory is designed for distributed networking environments, but in practice in NZ, gets implemented in small/medium business environments where its complexity and dependency on the underlying relatively unstable Windows 2000 OS make it more of a hindrance than a help.
Major flaw: ALL member servers which carry an AD replica must be Windows platforms. Contrast with NDS/eDirectory where a master replica can exist on any supported OS - These include; Netware, Linux, Windows, and Solaris.
Active Directory features include (from MS themselves, so read like an advertising blurb):
Active Directory is an essential component of the Windows 2000 architecture. It presents organizations with a directory service designed for distributed computing environments. Active Directory allows organizations to centrally manage and share information on network resources and users while acting as the central authority for network security. In addition to providing comprehensive directory services to a Windows environment, Active Directory can (through LDAP and native windows integration) be a consolidation point for isolating, migrating, centrally managing, and reducing the number of directories that companies require.