User stories allow the functionality of a product or service to be expressed as written descriptions of an experience as seen from the users perspective. The writing of user stories creates a list of design and development tasks to complete in order to deliver any required functionality.
User stories are distinct and isolated tasks that can be addressed individually or part of a series of separate tasks making up a larger user journey (Epics). User stories always contains an actor (the user identified in the story), a narrative (what the users is expected to do), and a goal (why the users wants to do it). They are written in a straightforward manner that should make sense from the user’s perspective. They should not contain technical specifications or system requirements.
As part of an agile design and development process, user stories populate a backlog of tasks that are prioritised and delivered in sprints. Each story is tagged and assigned a difficulty rating that can be used to determine task burndown rate.
User stories are written by members of the design team on a project. They are often captured in team exercises on paper, then moved and kept track of on project management software.
User Stories Applied, by Mike Cohn
How to write good user stories