User Story

A user story is a chunk of functionality (some people use feature) that is of value to the business. A story should be understandable to business owners and developers, testable, valuable to the business owner, and small enough so that a developer can build several per week. A story is a simple way to chop up what the system needs to do so it can be delivered incrementally if necessary.

In User Stories Applied Mike Cohn suggests a more formal approach to writing user stories. He suggests the format:

As a (role) I want (something) so that (benefit).

For example, a user story could be rewritten as

As a Student I want to purchase a parking pass so that I can drive to school.

This approach helps you to think about who a certain feature is built for and why.

When is a story considered to be completed?
The story is considered complete when the function of that story is demonstrated to pass the defined Acceptance Tests.