The diagram shows the processes within the system,which are visible to the outside world. Basically these are the Use Cases,the Actors of the system being modelled,and the relationships between them.

A use case diagram is a picture of some or all of the actors and their interactions with the system. Each system will have a main use case diagram showing the system boundary and the major functions provided by the system. There will be a communication association between the Actor and the use case.

A simple example use case diagram