A user story is a tool to describe the product functionality, but it is less useful suited to describe in detail the user interaction. Agile scenarios and storyboards are tools you need to describe the interaction steps. In his post, Roman Pichler what scenarios and storyboards are, how they can be used effectively in an Agile approach and how they relate to user stories.
In a traditionnal Agile requirements approach, epic are used as high leve user stories. They can be compared to a scenario that hides all the specifics of the user interaction. Detailed user stories are similar to individual steps in a scenario and describe a specific part of the product functionality. Scenarios and boards complement user stories and help explore the riskier one. They help also to discover new user stories and capture the relationship between stories.
Read the complete blog post on http://www.romanpichler.com/blog/agile-product-management-tools/agile-scenarios-and-storyboards/
