Depicts the typical communication between external users and the system. The emphasis is on what a machine does rather than how it works it. A use case is a summary of scenarios for a single goal or task. An actor is responsible for starting a task. The connection between use case and actor is a communication association.
Defining use cases is one of the basic tasks of the elaboration phase of RUP. In its simplest usage, you create a use case by talking to your users and defining the several things they might want to do with the system.