What is a Sequence Diagram in UML?
Sequence diagram is the most common kind of interaction diagram which focuses on the message interchange between a number of lifelines.
Sequence diagram describes an interaction by focusing on the sequence of messages that are exchanged, along with their corresponding occurrence specifications on the lifelines. It is a good way to visualize and validate various runtime scenarios. These can help to predict how a system will behave and to discover responsibilities a class may need to have in the process of modeling a new system.