Sequence diagram and Collaboration are both Interaction Diagrams that main difference b/t the two is that Sequence diagram shows Interaction of objects w.r.t. time while collaboration diag. does not.Also with collaboration diag it is more easy to depict that with how many objects does an object interact. State diagram shows the states of an object within many use cases while activity diag shows states of many objects in many use cases... Pls correct me if i am wrong. Bye, Poonam.