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)