Active Directory (AD) is a directory service that Microsoft developed for
Windows domain networks and included in most Windows Server operating systems
as a set of processes and services.

An AD 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. For example, when a user
logs into a computer that is part of a Windows domain, Active Directory checks
the submitted password and determines whether the user is a system
administrator or normal user.

Active Directory makes use of Lightweight Directory Access Protocol (LDAP)
versions 2 and 3, Microsoft's version of Kerberos, and DNS.