Systems always have users. Users in the classic sense are people who use the system, but users can also be other systems that trade information, or even devices.

In use case diagrams, people, systems, and devices are all referred to as actors.
The icons to model them vary, but the concept remains the same. An actor is a role that an external entity plays in relation to the system. To reiterate, an actor is a role, not a person or a job.

For example, an actor may be the role of a bank supervisor who uses a system to authorize a transaction that requires supervisor authority. The supervisor may also cover for a teller during a break, thus functioning in both roles. Likewise, many people can function in the same role, banks can have numerous tellers. Using roles helps keep us focused on how the system is being used rather than on the current organization of the company into job titles and responsibilities. How many times have you rewritten an application because a company reorganized its staff or departments?

So how do you spot actors? Listen to descriptions of the system. Listen for the roles people perform when using the system, typically, they will be the subject of a sentence describing how they use the system. When multiple people perform the same function, try to name the role they all share when performing the particular function. Throughout the modeling effort, the vocabulary of the user will reveal most of the key elements of the model.
Watch for how parts of speech translate into model elements; actors typically show up as the subject in sentences describing systems.
Step through the process of adding actors to the use case diagram:

Add Use Case Actors
In the next lesson, how to specify use cases and the features that actors use will be discussed.