Defining the system boundary determines what is considered external or internal to the system.
An actor represents a role played by an outside object. One object may play several roles and, therefore, is represented by several actors.
A communicates relationship illustrates the participation of the actor in the use case.
A use case is a set of events that occurs when an actor uses a system to complete a process. Normally, a use case is a relatively large process, not an individual step or transaction.