User story
A short, plain-language description of a feature from the perspective of the person who wants it, commonly used as a Product Backlog item format.
What it means
User stories are not defined by the Scrum Guide, which only speaks of Product Backlog items. The common format, 'As a [user], I want [goal], so that [reason]', comes from Extreme Programming and popular Agile practice.
A well-written story is small enough to fit in a Sprint, independent, negotiable, valuable, estimable, and testable, criteria often remembered by the acronym INVEST.
Common mistakes
Stories usually pair with acceptance criteria that define when the story is considered correctly implemented, distinct from but complementary to the team's Definition of Done.
Example
'As a returning customer, I want my saved address pre-filled, so that checkout is faster' is a typical user story.