Active Directory naming contexts (directory partitions)

Active Directory is the central repository in which all objects in an enterprise and their respective attributes are stored. It is a hierarchical, multi-master enabled database, capable of storing millions of objects. Those objects areorganized as a treeand stored into sections, called naming contexts or directory partition.

Each directory partition contains a hierarchy (subtree) of directory objects. At minimum, each domain controller stores three NCs in its local copy of the Active Directory database.
Active Directory naming contexts

Configuration NC contains objects about sites, services, and directory partitions. Every domain controller in the forest has a replica of the same configuration partition.

Configuration NC

Schema NC contains the Schema container, which stores class and attribute definitions for the forest. Every domain controller in the forest has a replica of the same schema partition

Domain NC : The domain partition contains the directory objects, such as users and computers, associated with the local domain. Each domain controller stores a full replica of the domain partition for its local domain, but does not store replicas of the domain partitions for other domains