Active Directory (AD) comes with most Microsoft Windows servers and is an advanced, hierarchical network directory service. First introduced in Windows 2000 this service allows its users to manage permissions and access network resources.
Active Directory is a domain-based network with Active Directory Domain Services (AD DS) being the key element of every Windows domain network. Active Directory uses Lightweight Directory Access Protocol (LDAP) that allows assigning workgroups with domain names. Windows, Mac, Unix or Linux client should be LDAP-compliant to access them. Active Directory can determine whether users who log in to computers that are a part of Windows domain are system administrators or regular users.
All in all Active Directory is used for data storage and management, offers authentication and authorization mechanisms, and provides a framework for other related services implementation.