Active Directory database is logically separated into directory partitions:
Each partition is a unit of replication, and each partition has its own replication topology. Replication occurs between replicas of directory partition. Minimum two directory partitions are common among all domain controllers in the same forest: the schema and configuration partitions. All domain controllers which are in the same domain, in addition, share a common domain partition.
Only one schema partition exists per forest. The schema partition is stored on all domain controllers in a forest. The schema partition contains definitions of all objects and attributes that you can create in the directory, and the rules for creating and manipulating them. Schema information is replicated to all domain controllers in the attribute definitions.
There is only one configuration partition per forest. Second on all domain controllers in a forest, the configuration partition contains information about the forest-wide active directory structure including what domains and sites exist, which domain controllers exist in each forest, and which services are available. Configuration information is replicated to all domain controllers in a forest.
Many domain partitions can exist per forest. Domain partitions are stored on each domain controller in a given domain. A domain partition contains information about users, groups, computers and organizational units. The domain partition is replicated to all domain controllers of that domain. All objects in every domain partition in a forest are stored in the global catalog with only a subset of their attribute values.
Application partitions store information about application in Active Directory. Each application determines how it stores, categorizes, and uses application specific information. To prevent unnecessary replication to specific application partitions, you can designate which domain controllers in a forest host specific application partitions. Unlike a domain partitions, an application partition cannot store security principal objects, such as user accounts. In addition, the data in an application partition is not stored in the global catalog.