System behavior is what a system does when responding to external events
use case outwardly visible and testable system behavior
Actor whoever or whatever (person, machine, etc) that interacts with a use case
Actor receives a useful resuft
Use case represents a complete unit of functionality of value to an actor
There may be some use cases that do not directly interact with actor
Ln many instances, a function requirement maps directly to a use case
Use Case Diagram is a visual representation of actors and use cases together with any additional and specifications