A class diagram is a type of static structure diagram in the Unified Modeling Language (UML) that describes the structure of a system by showing the systems classes, their attributes, operations (or methods), and the relationships among objects. It is the main building block of object-oriented modeling and can be used for general conceptual modeling of the structure of an application, as well as for detailed modeling, translating the models into programming code. Class diagrams can also be used for data modeling.

In a UML class diagram, classes are represented with boxes that contain three compartments: the top compartment contains the name of the class, the middle compartment contains the attributes of the class, and the bottom compartment contains the operations (or methods) of the class. Classes may be involved in one or more relationships with other classes, which can be one of the following types: inheritance (or generalization), simple association, or aggregation.

Class diagrams can be used in different development phases of a software development lifecycle and can be modeled in three different perspectives (levels of detail) progressively as we move forward.

In summary, a class diagram in UML is a graphical notation used to construct and visualize object-oriented systems by describing the structure of a system through its classes, attributes, operations, and relationships among objects.