User story is one or more sentences that captures what a user does or needs to do as part of his job function. User stories are used with agile software development methodologies as the basis for defining the functions a business system must provide, and to facilitate requirements management. It captures the ‘who’, ‘what’ and ‘why’ of a requirement in a simple, concise way.
User stories are a quick way of handling customer requirements without having to create formalized requirement documents and without performing administrative tasks related to maintaining them. (Source: WikiPedia)
In software development and product management, a user story is an informal, natural language description of features of a software system. They are written from the perspective of an end user or user of a system, and may be recorded on index cards, Post-it notes, or digitally in project management software. Depending on the project, user stories may be written by different stakeholders like client, user, manager, or development team.