UML 2.0 Use Case Diagram Definition
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UML 2.0 use case diagrams extend UML 1.5 Use Case diagrams.
Definition
Use case diagrams describe required usages of a system, or what a system is supposed to do. The key concepts that take part in a use case diagram are actors, use cases, and subjects. A subject represents a system under consideration with which the actors and other subjects interact. The required behavior of the subject is described by the use cases.
Sample Diagram
The following diagram shows an example of actors and use cases for an ATM (Automated Teller Machine) system.
This example is taken from the UML 2.0 Superstructure Specification.