The Sequence diagram focuses is a type of Interaction diagram (which falls under the behavioral diagramming family) that focuses on the Message interchange between various Lifelines.
A Sequence diagram shows the interaction information with an emphasis on the time sequence. The diagram has two dimensions: the vertical axis representing time. and the horizontal axis representing the participating objects. The time axis could be an actual reference point, by placing the time labels as text boxes. The horizontal ordering of the objects is not significant to the operation, and you can rearrange them as necessary.