Use Case Diagram
Use Case Diagram describes about the functionalities of the system as well as tells about the users of the system.
Actors and Use Cases
Actors are the users of the system. Actors can be human users or other systems. Use cases are the functionalities of the system that users can use.
Communicates is an association between the actor and the use case.
When you want to make functionality optional to use case. For example, payment can be done by cheque, by cash or by credit card.
When one use case uses the functionality of other use case.