A use case is a description of a system's behaviour as it responds to a request that originates from outside of that system.
The use case technique is used in software and systems engineering to capture the functional requirements of a system. Use cases describe the interaction between a primary actor—the initiator of the interaction—and the system itself, represented as a sequence of simple steps. Actors are something or someone which exist outside the system under study, and that take part in a sequence of activities in a dialogue with the system, to achieve some goal: they may be end users, other systems, or hardware devices. Each use case is a complete series of events, described from the point of view of the actor.
According to Bittner and Spence, "Use cases, stated simply, allow description of sequences of events that, taken together, lead to a system doing something useful." Each use case describes how the actor will interact with the system to achieve a specific goal. One or more scenarios may be generated from each use case, corresponding to the detail of each possible way of achieving that goal. Use cases typically avoid technical jargon, preferring instead the language of the end user or domain expert. Use cases are often co-authored by systems analysts and end users. The UML use case diagram can be used to graphically represent an overview of the use cases for a given system and a Use-case analysis can be used to develop the diagram.
Within systems engineering, use cases are used at a higher level than within software engineering, often representing missions or stakeholder goals. The detailed requirements may then be captured in SysML requirement diagrams or similar mechanisms.
Reference:
http://en.wikipedia.org/wiki/Use_case
