A use case diagram at its simplest is a representation of a user's interaction with the system that shows the relationship between the user and the different use cases in which the user is involved. A use case diagram can identify the different types of users of a system and the different use cases and will often be accompanied by other types of diagrams as well. The use cases are represented by either circles or ellipses. - Wikipedia

I have made Use Case Diagram for following Topics.

1. Credit Card
2. e-Library online public access catalog (OPAC)
3. Online Shopping
4. Ticket vending machine
5. Point of Sale Terminal
6. Restaurant Business

A use case diagram is a dynamic or behavior diagram in UML. Use case diagrams model the functionality of a system using actors and use cases. Use cases are a set of actions, services, and functions that the system needs to perform. In this context, a "system" is something being developed or operated, such as a web site. The "actors" are people or entities operating under defined roles within the system.

Use Case Diagram are made available in this repository for reference and learning purposes.

Generally Students/Developers study about this under course Software Engineering.