Depicts object interactions over time: show a user or actor, and the objects and components they interact with in the execution of a use case. One sequence diagram typically represents a single Use Case 'scenario' or flow of events
A UML diagram showing the sequence of interactions among objects.
A UML Diagram showing the dynamic behavior as messages are passed between objects. Equivalent to a Collaboration Diagram. Which representation is appropriate depends on the problem under consideration. See Section 4.5, "Sequence Diagrams (To be written)" for more information.