What is Active Directory?---2017/05/01
Active Directory is a directory service. The term directory service refers to two things — a directory where information about users and resources is stored and a service or services that let you access and manipulate those resources. Active Directory is a way to manage all elements of your network, including computers, groups, users, domains, security policies, and any type of user-defined objects. It melds several NT services and tools that have functioned separately so far — User Manager for Domains, Server Manager, Domain Name Server — and provides additional functions beyond these services and tools.
Active Directory is built around Domain Name System (DNS) and lightweight directory access protocol (LDAP) — DNS because it is the standard on the Internet and is familiar, LDAP because most vendors support it. Active Directory clients use DNS and LDAP to locate and access any type of resource on the network. Because these are platform-independent protocols, Unix, Macintosh, and other clients can access resources in the same fashion as Windows clients.