ER Model vs EER Model

The EER model includes all of the concepts introduced by the ER model. Additionally it includes the concepts of a subclass and superclass (Is-a), along with the concepts of specialization and generalization. Furthermore, it introduces the concept of a union type or category, which is used to represent a collection of objects that is the union of objects of different entity types. - From Wiki http://en.wikipedia.org/wiki/Enhanced_entity%E2%80%93relationship_model