What are sequence diagrams?

Once you have some candidate objects, hierarchies, and behaviors identified, it can be useful to understand how the objects will interact with each other. The UML provides Sequence Diagrams to help this process. A sequence diagram shows example object instances horizontally with lifetimes that stretch vertically, with time flowing from top to bottom.
Sequence diagrams are often used in association with use cases where each scenario is depicted in a sequence diagram. They are helpful for understanding just how objects will interact with each other during particular cases.
The time line is represented by a vertical dashed line. The open box around the timeline indicates the object is in an active operation. The horizontal lines with arrows represent messages to other object. For example a student object send a login message to the web server when the student wants to login to the site to secure admission to a course.