Use Case Diagram :-

A Use Case is a thing an actor perceives from the system. A Use Case maps actors with functions. Importantly, the actors need not be people. As an example, a system can perform the role of an actor, when it communicate with another system.

In another angle a Use Case encodes a typical user interaction with the system. In particular, it:

• Captures some user-visible function.
• Achieves some concrete goal for the user.

A complete set of Use Cases largely defines the requirements for your system: everything the user can see, and would like to do. The below diagram contains a set of use cases that describes a simple login module of a gaming website.