A customer can do a number of simple transactions at an ATM, such as deposit money, transfer funds, or withdrawal cash. There also exist use cases for the activities between the ATM and the central bank system. Once the customer inserts their bank card and enters their PIN number, the central bank system needs to validate that customer and PIN number. If the customer wants to withdrawal cash, the central bank system must first update the customer's balance. Those outside parties are referred to as actors.
Actors are what interact with the system.
It is possible for user to be more than one actor. An actor represents a specific role a user can play. The same user can be both a cusotmer and administrator, but for the purposes of the model, the two roles are kept separate. A good use case will make sure that an actor can perform a task specific to that actor. Also, a use case can communicate only with actors outside the system, not with other use cases.