A use case is an activity within a system enabling a user to accomplish a goal. Pertinent to that goal, a use case diagram is a simple visual representation of user(s), system, use cases, and relationships among them.

Dating back to the early 1990’s, use case and use case diagramming are among the earliest constructs that the agile community of practice developed for modeling project deliverables, especially software systems and applications, at a high level. Here is an excellent 12′ introduction.