Active Directory domain controllers host exactly three predefined partitions. The configuration naming context is replicated to all domain controllers in the forest and contains information that is forest-wide, such as the site topology and LDAP query policies. The schema-naming context is also replicated forest-wide and contains all of the schema objects that define how data is stored and structured in Active Directory. The third partition is the domain naming context, which is replicated to all of the domain controllers that host a particular domain.
Windows Server 2003 introduces a new type of partition called an application partition, which is very similar to the other naming contexts except you can configure which domain controllers in the forest replicate the data contained within it. This capability gives administrators much more flexibility over how they can store and replicate data contained in Active Directory. If you need to replicate a certain set of data to only two different sites, you can create an application partition that will only replicate the data to the domain controllers in those two sites.
For more details on application partitions, see Chapter 3 in Active Directory, Second Edition (O’Reilly).
Application Partitions are new to Windows Server 2003, so this entire chapter applies only to Windows Server 2003 domain controllers. Windows 2000 domain controllers cannot host application partitions.