Actors are simply roles that people or objects play. The picture below is a Make Appointment use case for the medical clinic. The actor is a Patient . Actors are stick figures. Use cases are ovals.
Communications are lines that link actors to use cases.
A use case diagram is a collection of actors, use cases, and their communications. We've put Make Appointment as part of a diagram with four actors and four use cases. Notice that a single use case can have multiple actors. Use case diagrams are helpful in three areas. New use cases often generate new requirements as the system is analyzed and the design takes shape.