User stories serve the same purpose as use cases but are not the same. ... They are also used instead of a large requirements document. User Stories are written by the customers (you) as things that the system needs to do for them. They are similar to usage scenarios, except that they are not limited to describing a user interface. They are in the format of about three sentences of text written by the customer in the customers terminology without techno-syntax.
Another difference between stories and a requirements document is a focus on user needs. You should try to avoid details of specific technology, data base layout, and algorithms. You should try to keep stories focused on user needs and benefits as opposed to specifying GUI layouts.
To describe a program you would like to create.
To provide enough detail to make a reasonably low risk estimate of how long the story will take to implement in code.