While considering all these things we can say that replication is the process of sending update information for data that has changed in the directory to other domain controllers. When we go further details study about active directory replication, we can say that active directory database is replicated between domain controllers. The data replicated between controllers are also called "naming context".
So here only the changes are replicated once a domain controller has been established. In active directory replication we can define that it uses a multimaster model which means changes can be made on any controller and the changes are sent to all other controllers.
There are three main elements or components that are replicated between domain controllers such as,
Domain partition replica: It is the active directory database of a domain. Here each domain controller maintains a duplicate copy of its local domain partition replica. Domain controllers do not maintain copies of replicas from other domains. In any case when changers are made to the domain change is replicated to all domain controllers immediately.
Global catalog: In this case each forest contains only a single global catalog. First domain controller installed into a forest is the global catalog server. This will contains a partial replica of every object within each domain of the forest. So this allows easy and efficient searching for users, computers, resources and other objects.
Schema: When we go through the schema, we can say that every domain controller in a forest has a copy of the schema. So as with changes to the Active Directory database any changes to the Active Directory schema are replicated to all other domain controllers in the forest. However, fortunately schema is usually static so there is little replication traffic caused by schema changes.