Windows uses domains (Figure 1), which are groups of computers that are part of a network and share a common directory database. These domains are administered as a unit with common rules and procedures. Each domain has a unique name, and users log into specified domains. The standard file structure in a Microsoft domain has a flat structure, and it is often difficult to locate resources on the network (Figure 2).
Figure 1: Windows domain
Active Directories is a directory service which enhances the operation of the directory structure by storing information about ob-jects on a network. It then makes this information available to users and network administrators, which allows users of the network access to permitted resources from any-where on the network using a single logon process. It also provides a hierarchical view of the network and a single point of administration for all network objects (which was the default in Windows NT).
Figure 2: Flat v. hierarchical structure
Active Directory domains can be administered as a single entity, with a common directory database, security policies, and security relations with other interconnected domains. A domain administrator maintains all user and group accounts. An Active Directory forest can then be created with more than one physical locality.
Active Directory requires a domain controller which is a host that run Active Directory, and manages access to the network, such as logging into the domain, authentication, and ac-cesses to directories and shared resources.
In a similar way to NDS, Active Directory has a directory partition, which is a contiguous subtree of a directory that forms a unit of replication. The domain controller stores three partitions for its own domain (and no other domain), these are:
Schema. This is the object classes and attributes contained in Active Directory. Configuration. This identifies the domain controllers, replication topology and other information about the domain controllers within a specific implementation of Active Directory. One or more domains. These contain the actual directory object data.
Every domain controller within the domain stores the same partition information has the same information (Figure 2). This is achieved with replication, which copies data from a data store or file system to multiple computers in order to synchronize their data. Active Directory sup-ports a multimaster replication of the directory between domain controllers within a given domain, and make the replicas of the directory on each domain controller writable. This facility allows updates to be applied to any replica of a given domain, and the replication service automatically copies of the changes from replica which has been changed to all other replicas. A replica is a which is a folder within a replica set, which is one or more shared folders that are contained within the replication area. The replication topology defines how the physical devices are connected between replicas. DFS only provides for logical descriptions of interconnections.
Figure 2: Active Directory domain controller
Here's a quick animation of the domain controller:
WJ, "Handbook of the Internet", Kluwer, 2002.