Active Directory
is domain controller authenticates and authorizes all users and computers in a Windows domain type network—assigning and enforcing security policies for all computers and installing or updating software.
As a directory service, an Active Directory instance consists of a database and corresponding executable code responsible for servicing requests and maintaining the database. The executable part, known as Directory System Agent, is a collection of Windows services and processes that run on Windows 2000 and later.[1] Objects in Active Directory databases can be accessed via LDAP, ADSI (a component object model interface), messaging API and Security Accounts Manager services.