Use case diagrams illustrate, at a very high level, the relationship between “actors” (whoever or whatever interacts with the system being designed) and the system itself. Each use case describes the sequence of events of an actor (human or system) using the system to complete some process. They provide a natural high-level view of the intended external functionality of the system that is understandable by engineers and non-engineers alike.
A use case is a generic description of an entire transaction involving several elements. It can also describe the behavior of a set of elements, such as an organization. A use-case model thus presents a collection of use cases and is typically used to specify or characterize the behavior of a whole system or a part of a system together with one or more external actors that interact with that system. An individual use case can have a name (although it is typically not a simple name). Its meaning is often written as an informal text description of the external actors and the sequences of events between elements that make up the transaction.
Note that use case diagrams treat the system itself as transparent, because the internals of the system are not of interest in this type of diagram. Use case diagrams capture key uses of the system and are therefore an excellent means of illustrating and explaining project requirements at a very high, non-technical level.
The following sections describe in more detail how to use case diagrams.