UML

Use Case Diagrams

Use case diagrams are behavior diagrams used to describe a set of actions (use cases) that some system or systems (subject) should or can perform in collaboration with one or more external users of the system (actors). Each use case should provide some observable and valuable result to the actors or other stakeholders of the system.

Note, that UML 2.4 specification also describes use case diagrams as a specialization of class diagrams, and class diagrams are structure diagrams.

Use case diagrams are in fact twofold - they are both behavior diagrams (because they describe behavior of the system), and they are also structure diagrams - as a special case of class diagrams where classifiers are restricted to be either actors or use cases related with association.

Use case diagrams are used to specify:

(external) requirements on a subject, required usages of a system - to capture what a system under construction is supposed to do;
the functionality offered by a subject – what system can do;
requirements the specified subject poses on its environment - by defining how environment should interact with the subject so that it will be able to perform its services.

Major elements of the use case diagram are shown on the picture below.
UML use case diagram major elements.

Use case diagram major elements.

You can find some examples of use case diagrams here:

Business model - Airport check-in and security screening
Business model - Restaurant
e-Library online public access catalog use cases
Point of Sales Terminal
Use cases for a retail website
Credit card processing system
ATM use cases
Hospital Management