Active Directory is a directory service that stores information about your organization such as user and computer accounts, certificates,policies. Active Directory stores the information in a file called ntds.dit.
Active Directory can be divided into two main categories, logical and physical components.The logical components help you structure the identity infrastructure.The logical structure contains two types of objects either container objects or leaf objects.Then we have forests that is the top level in the hierarchy.
The domain contains the logical components to achieve the administative goals of the organization and by default it becomes the security boundary for the objects inside it.A domain tree is a collection of domains that reflects the organizations structure.
OUs help group objects on a smaller scale within the domain.The most common way is to group objects that have similar security and administrative requirements together.For example the sales department uses common shared printers and folders so the security requirements for data and networks are similar, so now we can create a OU named sales and group the sale department users in that and apply security policies at the OU level instead of user level.
The domain controller is computer that runs the Windows Server operating system and holds the Active Directory Domain Services role, it can be a physical or virtual server.It holds the directory partition that will be replicated to all other domain controllers in the same domain. The global catalog server holds the full writeable copy of objects in its host domain and the partial copy of the objects in other domains in the same forest.The partial replica contains a copy of every object in the forest and the most commonly used attributes in queries.