User stories are written by the customer with designers.
from http://www.agile-software-development.com
and http://www.extremeprogramming.org/rules/userstories.html
User Stories are a simple way of capturing user requirements throughout a project - an alternative to writing lengthy requirements specifications all up-front. They are written in the everyday language of the user. They are used for the specification of requirements and lead to a release plan.
Benefits
Challenges