A use case diagram is a graphical depiction of a users possible interactions with a system. It is a way to summarize details of a system and the users within that system, and is generally shown as a graphic depiction of interactions among different elements in a system. Use cases specify the expected behavior of a system from the end users perspective, and not the exact method of making it happen. A use case diagram illustrates a set of use cases for a system, i.e. the actors and the relationships between the actors and use cases. The actors are usually individuals involved with the system defined according to their roles. The use cases are represented by either circles or ellipses, and the actors are often shown as stick figures. A use case diagram can help provide a higher-level view of the system, and is an effective technique for communicating system behavior in the users terms by specifying all externally visible system behavior. It is usually simple and does not show the detail of the use cases. Use case diagrams are employed in UML (Unified Modeling Language), a standard notation for the modeling of real-world objects and systems. The main purpose of a use case diagram is to portray the dynamic aspect of a system and to accumulate the systems requirements, which includes both internal as well as external influences. The diagram models the tasks, services, and functions required by a system/subsystem of an application. The following are the purposes of a use case diagram:

• Represent the goals of systems and users.
• Specify the context a system should be viewed in.
• Specify system requirements.
• Provide a model for the flow of events when it comes to user interactions.
• Provide an outside view of a system.
• Show external and internal influences on a system.

To draw a use case diagram, it is essential to analyze the whole system before starting and then identify the systems functionalities. Once every single functionality is identified, they are then transformed into the use cases to be used in the use case diagram. Once both the actors and use cases are enlisted, the relation between the actor and use case/system is inspected.