Here is a scenario for a medical clinic. A patient calls the clinic to make an appointment for a yearly checkup. The receptionist finds the nearest empty time slot in the appointment book and schedules the appointment for that time slot. A use case is a summary of scenarios for a single task or goal. An actor is who or what initiates the events involved in that task.
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.