A Class Diagram is a visual representation used in software engineering to illustrate the structure of a system. It shows the different classes in the system, their attributes, methods, and the relationships between them. This helps developers understand how different parts of the system interact and work together. In a Class Diagram, each class is represented as a rectangle divided into three sections: the top section contains the class name, the middle section lists its attributes, and the bottom section shows its methods. Relationships such as inheritance and associations are depicted using lines and arrows, providing a clear overview of the system's architecture.