A sequence diagram is a visual representation of a series of interactions between the objects in your system. Unlike a class diagram, which is a representation of a static model of the system, a sequence diagram is a representation of the dynamic interaction of your system, serialized over time. For more information about sequence diagrams, see Sequence diagrammer.
A method call appears within a sequence diagram and represents a message send from one object to another, and the passing of control from the sender to the receiver.
A method instance (or activation) appears within a sequence diagram and represents the active execution of a method, beginning with a message send and ending with a return.
A method return appears within a sequence diagram and represents the return of control from a method that has finished executing.