Class diagrams are the blueprints of your system or subsystem. You can use class diagrams to model the objects that make up the system, to display the relationships between the objects, and to describe what those objects do and the services that they provide. Class diagrams are useful in many stages of system design.
They are used to show the different objects in a system, their attributes, their operations, and the relationships among them. The following figure is an example of a simple class: Simple class diagram with attributes and operations.
