An Object diagram is closely related to a Class diagram, with the distinction that it depicts object instances of Classes and their relationships at a point in time. This might appear similar to a Composite Structure diagram, which also models run-time behavior; the difference is that Object diagrams exemplify the static Class diagrams, whereas Composite Structure diagrams reflect run-time architectures different from their static counterparts. Object diagrams do not reveal architectures varying from their corresponding Class diagrams, but reflect multiplicity and the roles instantiated Classes could serve.
They are useful in understanding a complex Class diagram, by creating different cases in which the relationships and Classes are applied. An Object diagram can also be a kind of Communication diagram, which also models the connections between objects, but additionally sequences events along each path.
Example Diagram: Example Object Diagram
Select Object diagram elements and connectors from the Object pages of the Toolbox.