Definition: "An actor represents a coherent set of roles that users of use cases play when interacting with these use cases. Typically, an actor represents a role that a human, a hardware device, or even another system plays with a system."
Example: If you work at El Wombat you might be a Cashier. If you happen to eat there as well you might also play the role of Customer.
An instance of an actor represents an individual playing a role that interacts with the system. It is important to acknowledge that actors are not actually part of the system, but live outside it.
Actors are depicted as stick figures and can be specialized from a general actor by using generalization relationships. For instance, in Figure-1 Engineer and Sales are special types of the more general Employee. As such they exhibit all the behaviors of an employee and add some specific nuances specific to "Engineer-ness" or "Sales-ness". Customer has no specialized types.
Figure 1 : Example Actors