UML 2 in a Nutshell: Use Case Diagrams
Source: O'Reilly Media
Use cases are a way to capture system functionality and requirements in UML. Use case diagrams consist of named pieces of functionality (use cases), the persons or things invoking the functionality (actors), and possibly the elements responsible for implementing the use cases (subjects). Use cases represent distinct pieces of functionality for a system, a component, or even a class. Each use case must have a name that is typically a few words describing the required functionality, such as View Error Log. UML provides two ways to draw a use case.