The Trouble with User Stories

Are you sure that you do know what the user story is? It is a vital part of the Agile manifesto or scrum guide, but what is it? Why is it so important? A dozen definitions say that user story can be a feature, high-level requirement, a unit of work, functionality, capability, or anything else related to the project. These definitions come up from Agile gurus, influencers, and bloggers – but no one defines what user story is.