Windows 2000 and Windows Server 2003 Active Directory uses a database set of rules called “Schema”. The Schema is defines as the formal definition of all object classes, and the attributes that make up those object classes, that can be stored in the directory. As mentioned earlier, the Active Directory database includes a default Schema, which defines many object classes, such as users, groups, computers, domains, organizational units, and so on. These objects are also known as “Classes”. The Active Directory Schema can be dynamically extensible, meaning that you can modify the schema by defining new object types and their attributes and by defining new attributes for existing objects. You can do this either with the Schema Manager snap-in tool included with Windows 2000/2003 Server, or programmatically.