What is difference between object diagrams and class diagrams?
A class diagram is a graph of Classifier elements connected by their various static relationships. Note that a class diagram may also contain interfaces, packages, relationships, and even instances, such as objects and links. Perhaps a better name would be static structural diagram, but class diagram is shorter and well established.
An object diagram is a graph of instances, including objects and data values. A static object diagram is an instance of a class diagram; it shows a snapshot of the detailed state of a system at a point in time. The use of object diagrams is fairly limited, mainly to show examples of data structures.
The basic idea is that class diagrams focus on classes and object diagrams focus on objects, but it is possible to mix classes and objects on the same diagram for various purposes, so the separation is not rigid. Class diagrams show the logical, static structure of a system and are central the Unified Modeling Language. Object diagrams play a smaller role in UML. They are used primarily for documenting test cases and scenarios, and for discussing examples.