Concept

A sequence diagram is a model describing how groups interact over time. It is an interaction diagram that details how operations are carried out: what messages are sent and when.

You use the sequence diagram model to describe the aspects of your system that change over time. These are events that mark changes, sequences of events and so forth. The time sequence interaction is shown in the sequence diagram and consists of two dimensions. The vertical dimension (time) and the horizontal dimension (different objects).
UML sequence diagrams can be used to:

explore your design
feel which classes are going to be complex
validate the logic and completeness of a usage scenario
detect bottlenecks within an object-oriented design

The elements of the sequence diagrams are available in the tool bar:

New Object can be created by selecting the icon in the toolbar. Allows you to add a new instance within the diagram.

New Actor can be created by selecting the icon in the toolbar

Message can be created by selecting the icon in the toolbar. Each message represents a method call.

Self Message can be created by selecting the icon in the toolbar. A reflexive message is a message sent by an instance to itself.