Use Case Diagrams is an overview graphical representation of the functionality in a system. It is used in the analysis phase of a project to specify the system to be developed.
In Use Case Diagrams the whole system is defined as ACTORS, USE CASES and ASSOCIATIONS, the ACTORS are the external part of the system like users, computer software & hardware, USECASES is the behavior or functionality of the system when these ACTORS perform an action, the ASSOCIATIONS are the line drawn to show the connection between ACTORS and USECASES. One ACTOR can link to many USECASES and one USECASE can link to many ACTORS.