User story is one of the primary development artifact for the XP project teams. A user story contains just enough information so that development team can reasonably give estimate about completing, tester can discuss how it will validated and customer can see its value.
One of the common question that we hear most of the time is, how user stories are different from use cases. User stories are much simpler than use cases. User stories are very easy to create, discuss and develop. They also do not contain any technical details.