User stories
User stories are short descriptions of features your customer would like to see in their software. Think of them as Agile requirements. They are written on index cards, and contain no more than a couple days work.
Now you can't write everything down on an index card and that's by design. User stories were made to encourage face-to-face communication and to rely less on documentation.
User stories are important because they form the basic units of work in Agile Planning. They are how Agile teams track work, measure progress, and set expectations around what needs to be done.