User Story

User stories are short, simple description of a feature told from theperspective of the person who desires the new capability, usually a user or customer of the system. They typically follow a simple template:

As a <type of user>, I want <some goal> so that <some reason>.

User stories are often written on index cards or sticky notes, stored in a shoe box, and arranged on walls or tables to facilitate planning and discussion. As such, they strongly shift the focus from writing about features to discussing them. In fact, these discussions are more important than whatever text is written.

Example:

One of the benefits of agile user stories is that they can be written at varying levels of detail. We can write a user story to cover large amounts of functionality. These large user stories are generally known as epics. Here is an epic agile user story example from a desktop backup product:

• As a user, I can backup my entire hard drive.

Because an epic is generally too large for an agile team to complete in one iteration, it is split into multiple smaller user stories before it is worked on. The epic above could be split into dozens (or possibly hundreds), including these two:

• As a power user, I can specify files or folders to backup based on file size, date created and date modified.
• As a user, I can indicate folders not to backup so that my backup drive isn't filled up with things I don't need saved.

How is detail added to user story

Detail could be added to that user story example by adding the following conditions of satisfaction:

• Make sure it works with major retail holidays: Christmas, Easter, President’s Day, Mother’s Day, Father’s Day, Labor Day, New Year’s Day.
• Support holidays that span two calendar years (none span three).
• Holiday seasons can be set from one holiday to the next (such as Thanksgiving to Christmas).
• Holiday seasons can be set to be a number of days prior to the holiday.

http://www.mountaingoatsoftware.com/agile/user-stories#resources