The Active Directory schema is the set of definitions that defines the kinds of objects, and the

types of information about those objects, that can be stored in Active Directory. The

definitions are themselves stored as objects so that Active Directory can manage the schema

objects with the same object management operations used for managing the rest of the

objects in the directory.

There are two types of definitions in the schema: attributes and classes. Attributes and

classes are also referred to as schema objects or metadata.

Attributes are defined separately from classes. Each attribute is defined only once and can be

used in multiple classes. For example, the Description attribute is used in many classes, but is

defined once in the schema, assuring consistency.