What is an Actor in a Use Case Diagram and what types of actors exist?

An actor is any entity outside the system that plays a role and triggers actions in it — and it need not be human: users, external systems, and devices all count.

An actor is an external interface to the system. The key insight beginners miss is that an actor is a role, not a specific person — and not necessarily a person at all.

Types of actors:

Key points:

• Represented as a stick figure with a label
• Always exists outside the system boundary
• Activates and influences flows of actions
• Can be generalized/specialized (inheritance)

Tip: Actor = Any entity that "acts" on or with the system from the outside.