The actor in a use case diagram is not necessarily a person—it is rather the user of the system. It could be a person, another system, or even another module of the system. A visual representation of an actor is given in the following diagram:

An actor is responsible for providing an input. It gives instructions to the system and the system works accordingly. Every action an actor does has a purpose. A use case diagram shows us what an actor can do and what the expectations of the actor are.