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, which are blueprints for creating objects, along with their attributes (properties) and methods (functions). Each class is represented as a rectangle divided into sections, making it easy to see how they relate to one another. In a class diagram, relationships between classes are depicted using lines and arrows. For example, a relationship can indicate that one class inherits from another, or that two classes are associated in some way. This helps developers understand how different parts of the system interact, making it easier to design and maintain software.