Extreme Programming (XP), one of many Agile methods, introduced the practice of User Stories.  They are short descriptions of functionality the system should provide.  The initial descriptions are often created or written by the users or customers of the system.  The user stories are used during the iterative planning and development cycles to determine a unit of work and estimate how long it will take.  Most practitioners strive to make a user stories fit into a 1-3 week span of development

User stories do not end at a 2-3 line description of functionality.  Each user story consists of three parts:

• The Card: Named for the standard index cards on which a user story is often captured.  The cards are used for planning the work that will be completed during each iteration of development.
• The Conversations:  Discussions about each user story are had with the users/customers of the system to flesh out details.  The conversations are captured and documented as part of the user story.
• The Confirmation: Test scenarios that capture details about the user story that can be used to verify that the user story has been successfully implemented.

Using these three parts, the goal of the user story is to provide enough detail that a developer can understand what needs to be done while providing a means to verify that they have achieved the goal.  User stories are often equated to a single use case scenario, such as the main use case scenario or an alternative path.  The “Card” portion of the use case scenario is written in a manner similar to a brief description of a use case.