Computers that function as servers in a domain can be a member server or a domain controller. A member server belongs to a particular domain but does not authenticate the users of that domain. There is no Active directory data installed in it.
Domain controllers are servers responsible for allowing access to domain resources. It contains information on all user accounts, authenticates users and enforces security policy for a domain.
A domain controller has three directory partitions.
Domain partition – It contains users, computers, groups and other objects for a local domain. Each domain controller will have full replica of the domain partition.
Schema partition – The type of objects and attributes that can be created in a domain is completely controlled by the schema. Nevertheless, the schema is extensible – it supports the creation of new types of objects and attributes.
Configuration partition – The configuration partition contains the replication topology and other configuration information that needs to be replicated across the forest. Every domain controller will have the same replica of schema and configuration partition.