Sequence diagrams show the communication behavior between parts of a system. A Sequence diagram can be a behavior specification or a trace of actual runtime communication.
A sequence diagram describes a pattern of interaction among objects, arranged in a chronological order. It shows the objects participating in the interaction by their "lifelines" and the messages that they send to each other.
In most cases, we use a sequence diagram to illustrate use-case realizations. That is, realizations show how objects interact to perform the behavior of all or part of a use case. One or more sequence diagrams may illustrate the object interactions that enact a use case. A typical organization is to have one sequence diagram for the main flow of events and one sequence diagram for each independent sub-flow of the use case.
Sequence diagrams are particularly important to designers because they clarify the roles of objects in a flow and provide basic information for determining class responsibilities and interfaces.