A class diagram is a picture for describing generic descriptions of possible systems. Class diagrams contain classes and object diagrams contain objects, but it is possible to mix classes and objects when dealing with various kinds of metadata, so the separation is not rigid.

Class diagrams are more prevalent than object diagrams. Class diagrams contain icons representing classes, interfaces, and their relationships. You can create one or more class diagrams to depict the classes at the top level of the current model; such class diagrams are themselves contained by the top level of the current model. You can also create one or more class diagrams to depict classes contained by each package in your model; such class diagrams are themselves contained by the package enclosing the classes they depict; the icons representing logical packages and classes in class diagrams.