Domain: A domain is a central part of AD’s structure. It’s a logical group of network objects, like users, computers, and devices, that share the same AD database. A domain is managed by a domain controller.

Domain Controller (DC): A server that responds to security authentication requests within a Windows Server domain. It stores user account information, authenticates users, and enforces security policies for a domain.

Organizational Unit (OU): These are containers within a domain that can hold users, groups, computers, and other OUs. They provide a way to organize directory objects into logical hierarchies, offering more granular control over the management of these objects.

Global Catalog (GC): This is a distributed data repository that contains a searchable, partial representation of every object in every domain in a multi-domain AD forest. It helps in locating objects from any domain without needing their domain name.

Schema: The schema is a component of AD that defines all the objects and attributes that the directory service can manage. It determines the kinds of objects that can be stored in the directory and the attributes of those objects.