MicrosoftCorporation's directory service; basically, a schema for LDAP.

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 is designed to be a consolidation point for isolating, migrating, centrally managing, and reducing the number of directories that companies require. (Read: SinglePointOfFailure? :)