AD DS provides a distributed database that stores and manages information about network resources and application-specific data from directory-enabled applications. Administrators can use AD DS to organize elements of a network, such as users, computers, and other devices, into a hierarchical containment structure. The hierarchical containment structure includes the Active Directory forest, domains in the forest, and organizational units (OUs) in each domain. A server that is running AD DS is called a domain controller.
The forest acts as a security boundary for an organization and defines the scope of authority for administrators. By default, a forest contains a single domain, which is known as the forest root domain.
Additional domains can be created in the forest to provide partitioning of AD DS data, which enables organizations to replicate data only where it is needed. This makes it possible for AD DS to scale globally over a network that has limited available bandwidth. An Active Directory domain also supports a number of other core functions that are related to administration, including network-wide user identity, authentication, and trust relationships.