Data in a directory service is organized differently from traditional database application, which organizes data in tables that have a fixed number of columns. Each row in a table represents a single entity and contains a value for each column in the table. A directory service organizes data as a collection of objects. Each object is characterized by one or more object classes that define the values (or attributes) that the object defines. In addition, objects might be grouped into organizational units.

Because of these differences, there is no one-to-one mapping between rows/columns/tables and objects/attributes/object classes. The following table shows relationships among directory service, AR System , and typical database concepts.