A User Story is a small narrative "document" that describes
some function of a system, or a compact sequence of events of an actor
(an external agent) using a system to complete a process.
User stories capture who (actor) does what (interaction) with the system, for what purpose (goal), without dealing with system internals. A complete set of use cases specifies all the different ways to use the system, and therefore defines all behavior required of the system, bounding the scope of the system.