Structural class:
The structural class is important to the system administrator in that it is the only type from which new Active Directory objects are created. Structural classes are developed from either the modification of an existing structural type or the use of one or more abstract classes.
Abstract class:
Abstract classes are so named because they take the form of templates that actually create other templates (abstracts) and structural and auxiliary classes. Think of abstract classes as frameworks for the defining objects.
Auxiliary class:
The auxiliary class is a list of attributes. Rather than apply numerous attributes when creating a structural class, it provides a streamlined alternative by applying a combination of attributes with a single include action.
88 class:
The 88 class includes object classes defined prior to 1993, when the 1988 X.500 specification was adopted. This type does not use the structural, abstract, and auxiliary definitions, nor is it in common use for the development of objects in Windows Server 2003 environments.