What is the difference between Class Diagram and Object Diagram?
Wikipedia gives examples of both: Class Diagram Object Diagram
The differences are in the purpose, and what they show.
A Class Diagram will show what the Objects in your system consist of (members) and what they are capable of doing (methods).
In contrast, an Object Diagram will show how objects in your system are interacting with each other at some point in time, and what values those objects contain when the program is in this state.