You can think of an Actor as more of a role or an abstraction of "some entity of which details are unknown" rather than as a physical item. How you use Actors depends on what do you want to model in a system -- in some diagram, you might represent a user (human) as an Actor, but in the next diagram you could represent a computing system as an Actor. The diagram after that could represent a single Java class as an Actor. It's up to you.