The use case diagram describes the proposed functionality for a new system that will be designed, it is an excellent tool for surveying the functional requirements of the system. According to Ivar Jacobson, we can say that a use case is a "narrative document that describes the sequence of events of an actor who uses a system to complete a process". A use case represents a discrete unit of interaction between an actor (human, device, or other software[1]) and the system. A use case is a unit of significant work. For example, "login to the system", "log in to the system" and "create orders" are all use cases. Each use case has a description of the functionality that will be built into the proposed system. A use case can "include" another use case functionality or "extend" another use case with its own behavior.
Use cases are typically related to "actors". An actor is a human or machine entity that interacts with the system to perform significant work.
