A Use Case Diagram is a graphical representation of the interactions between a system and its users. It is used to identify the primary functions and possible scenarios of a system. It is used to identify the requirements of a system and to design the system in a structured way. The diagram consists of
From Wikipedia
A use case diagram is a graphical depiction of a user's possible interactions with a system. A use case diagram shows various use cases and different types of users the system has and will often be accompanied by other types of diagrams as well. The use cases are represented by either circles or ellipses. The actors are often shown as stick figures.