10. Use case diagram

Use case is one of the 'behaviour' class of diagrams defined in the UML specification.

The purpose of the Use case is to show the services and actions that a system can do in order to support the needs of an outside actor. This is done by diagramming a set of scenarios.

It is not a flow chart, it is not a sequence diagram because the diagram does not imply any sense of order to the services available within the system. It is a horizontal diagram in general, not a vertical one such as a typical flow chart.

The main constituents of an use case diagram are

Use cases. Drawn as a horizontal ellipse. An use case is a discrete service that the system is able to undertake in order to meet the requirements of an outside actor
Actor. An actor is a person, organisation or any other entity that is not part of the system but they do trigger an use case within the system
Associations. An association is a graphical way to show that an interaction of some kind exists between an actor and an use case. The end arrows means the same thing as in class diagrams
System boundary. This is a rectangular box that shows the boundary of a system. Actors only exist outside the boundary whereas use cases exist within the boundary. The boundary should be present in the top level diagram, but it does not need to be present in more internal detail diagrams
Packages. This is an optional feature of an use case diagram. A package allows you to group use cases into logical groups. They are depicted as a file folder
Includes. This term appears above an association line. It means that if case A happens then case B happens as well	<< includes >>
Extends. This term appears to denote that use case A is a special version of the more general use case B	<< extends >>

The next page is an shows a typical use case diagram.