A class diagram is a UML diagram that provides a graphical view of the classes, interfaces, and packages that compose a system, and the relationships between them.
You build a class diagram to simplify the interaction of objects in the system you are modeling. Class diagrams express the static structure of a system in terms of classes and relationships between those classes. A class describes a set of objects, and an association describes a set of links; objects are class instances, and links are association instances.
A class diagram does not express anything specific about the links of a given object, but it describes, in an abstract way, the potential link from an object to other objects.
The following example shows the process of product order and shipment in a class diagram.
