An Agile requirement, stated as a sentence or two of plain English. A user story is often expressed from the user’s point of view, and describes a unit of desired functionality.
A user story typically fits on a note card, and may include a title and a short description. The user story is not intended to capture every aspect of the feature to be developed; as described by proponents of Extreme Programming, it is meant to serve as a placeholder for a dynamic conversation between the product owner or customer, and the Agile team.
While a user story may take any form that works for the team, here is one format that has become popular:
As a [type of user] I want to [do something] so that I [get a benefit].
The product backlog is comprised of user stories. Large user stories are known as epics, which may be decomposed into user stories as they near implementation In the broadest terms, a user story that is appropriately sized for implementation is one that is a) implementable, and b) for which an acceptance test may be written.
The User Story originates with Extreme Programming, and replaces the more traditional Use Case, which was a more abstract and discursive model that usually focused more on the requirements for building the product and less on the user.