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Active Directory (AD)

A directory service created by Microsoft for Windows domain networks. Active Directory serves as a central location for network administration and security. It is responsible for authenticating and authorizing all users and computers within a network of Windows domain type, assigning and enforcing security policies for all computers in a network and installing or updating software on network computers. 

For example, when a user logs into a computer that is part of a Windows domain, Active Directory verifies that user's password and specifies whether he or she is a system administrator or normal user. Active Directory is LDAP compliant. Others active directories include Lotus Domino, SunOne/iPlanet Directory Server and Novell eDirectory.