What is Active Directory?

The main Active Directory service is Active Directory Domain Services (AD DS), which is part of the Windows Server operating system. The servers that run AD DS are called domain controllers (DCs). Organizations normally have multiple DCs, and each one has a copy of the directory for the entire domain. Changes made to the directory on one domain controller — such as password update or the deletion of a user account — are replicated to the other DCs so they all stay up to date. A Global Catalog server is a DC that stores a complete copy of all objects in the directory of its domain and a partial copy of all objects of all other domains in the forest; this enables users and applications to find objects in any domain of their forest. Desktops, laptops and other devices running Windows (rather than Windows Server) can be part of an Active Directory environment but they do not run AD DS. AD DS relies on several established protocols and standards, including LDAP (Lightweight Directory Access Protocol), Kerberos and DNS (Domain Name System).

It’s important to understand that Active Directory is only for on-premises Microsoft environments. Microsoft environments in the cloud use Azure Active Directory, which serves the same purposes as its on-prem namesake. AD and Azure AD are separate but can work together to some degree if your organization has both on-premises and cloud IT environments (a hybrid deployment).

The Active Directory database (directory) contains information about the AD objects in the domain. Common types of AD objects include users, computers, applications, printers and shared folders. Some objects can contain other objects (which is why you’ll see AD described as “hierarchical”). In particular, organizations often simplify administration by organizing AD objects into organizational units (OUs) and streamline security by putting users into groups. These OUs and groups are themselves objects stored in the directory.

Objects have attributes. Some attributes are obvious and some are more behind the scenes. For example, a user object typically has attributes like the person’s name, password, department and email address, but also attributes most people never see, such as its unique Globally Unique Identifier (GUID), Security Identifier (SID), last logon time and group membership.

Databases are structured, which means there is a design that determines what types of data they store and how that data is organized. This design is called a schema. Active Directory is no exception: Its schema contains formal definitions of every object class that can be created in the Active Directory forest and every attribute that can exist in an Active Directory object. AD comes with a default schema, but administrators can modify it to suit business needs. The key thing to know is that it’s best to plan the schema carefully up front; because of the central role AD plays in authentication and authorizations, changing the schema of the AD database later can dramatically disrupt your business.