An Active Directory server usually means a Windows Server computer running Active Directory Domain Services (AD DS) as a domain controller. AD DS keeps directory information about users, computers, and other network objects, and helps a domain authenticate users and control access.
What “Active Directory server” means
The formal name of the service is Active Directory Domain Services, or AD DS. The computer running that service is called a domain controller (DC). “Active Directory server” is common shorthand for an AD DS domain controller, rather than the official name of a separate server product.
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Microsoft describes Active Directory as a Windows implementation of a general-purpose directory service that uses LDAP as its primary access protocol. A directory is a hierarchical structure that stores information about objects on a network. Microsoft Learn’s AD DS overview covers the service, while the Microsoft Open Specifications glossary defines the terminology.
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What an AD DS domain controller does
AD DS stores information about network objects and makes that information available to clients and administrators. Depending on the organization, directory objects can include user and computer accounts, groups, printers, servers, and other shared resources.
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- Authentication: helps verify a user or computer’s identity when it accesses the domain.
- Access control: supports decisions about which domain resources an authenticated identity can use.
- Directory searches: lets clients and administrators find and query stored objects.
- Administration: provides a central place to manage domain identities and resources.
These are domain services; a domain controller is not simply a file server, even if the same Windows Server computer also performs other roles.
How clients find and use a domain controller
- DNS helps locate a controller. A domain-joined client uses DNS records to discover a domain controller for its domain. Domain controllers also use DNS to find one another. See Microsoft’s DNS and AD DS guidance and its overview of DNS in Windows and Windows Server.
- The client requests domain services. It contacts a domain controller for functions such as authentication and directory lookups. AD DS follows a client-server model, as described in the Microsoft Open Specifications introduction to AD DS.
- Administrators manage directory objects. Changes to accounts, groups, and other objects are made through directory administration tools and stored in the directory.
How domains, forests, and OUs fit together
AD DS has a logical hierarchy: a forest contains one or more domains, and domains can be organized into organizational units (OUs). Forests, domains, and OUs describe how directory information is structured; they do not specify how many physical servers an organization must deploy or where those servers must sit on its network. The number and placement of domain controllers are deployment decisions. Microsoft’s explanation of the Active Directory logical model distinguishes this structure from the physical deployment.
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Why a domain may have multiple domain controllers
A domain can use multiple domain controllers. Microsoft says each domain controller in a domain contains a complete copy of that domain’s directory information, and changes are replicated among them. This lets controllers serve the same domain directory across the deployment; DNS supports their discovery and communication.
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| Service | What it is for | Management and scope |
|---|---|---|
| Active Directory Domain Services (AD DS) | Traditional Windows domain services, including domain accounts and domain-controller-based authentication and access control. | Runs on domain controllers that the organization manages. |
| Active Directory Lightweight Directory Services (AD LDS) | An LDAP-accessible directory intended primarily to store application data; it does not host domain naming contexts. | A directory deployment distinct from AD DS domain services. |
| Microsoft Entra ID | Cloud identity and authentication for cloud resources. | A cloud identity service, not another name for an on-premises AD DS domain controller. |
| Microsoft Entra Domain Services | A managed domain service providing a subset of traditional AD DS capabilities. | Microsoft manages the domain-service infrastructure; capabilities differ from self-managed AD DS. |
These services are not interchangeable just because they all involve directories or identity. The choice depends on whether an environment needs traditional Windows domain capabilities and whether it wants to operate domain-controller infrastructure itself. Check Microsoft’s comparison of Microsoft directory-based services for current feature details before making an implementation decision.
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