Active Directory (AD) comes with its own lexicon of terms, some stemming from its International Standards Organization X.500 Directory Services heritage (which is also shared with other directories, such as Novell’s eDirectory service and Sun Microsystems’ Sun One Directory Server), and others that are exclusive to Microsoft. A high-level understanding of the following terms is critical to following any discussion of AD. (Some of these terms are also graphically illustrated in “Active Directory Cross-Forest Trust“.)

Active Directory Data Model

Objects, classes, and attributes. AD is a database of objects, where each object is a specific instance of a particular AD class, such as a user, group, computer, printer, or contact. Each object has many attributes that define it, such as the object’s name or security permissions.

Schema. A schema defines the classes for all objects in the database and the relationships between various classes. Unlike the Windows NT 4.0 directory, which has a fixed schema with only three classes-users, groups, and computers-the AD schema can be extended to add new classes or add new attributes to the default classes. For example, Exchange 2000 changes the AD user class to store information such as users’ e-mail addresses and mailbox servers.