Actor: People, systems, and devices that have a stake in the successful operation of the system.
System: Sets the context of the system in relation to the actors who use it and the features it must provide.
Use case: Identifies the key features of the system. Without these features, the system will not fulfill the user/actor requirements. Each use case expresses a goal that the system must achieve.
Association: Identifies an interaction between elements.
Use Case Diagrams
A use case diagram shows the relationship among use cases within a system or other semantic entity and their actors.
Use Case Diagram Semantics
Use case diagrams show actors and use cases together with their relationships. The use cases represent functionality of a system or a classifier, like a subsystem or a class, as manifested to external interactors with the system or the classifier.
A use case diagram is a graph of actors, a set of use cases, possibly some interfaces, and the relationships between these elements.
The relationships are associations between the actors and the use cases, generalizations between the actors, and generalizations, extends, and includes among the use cases. The use cases may optionally be enclosed by a rectangle that represents the boundary of the containing system or classifier.