User scenarios are the stories that a user acts out. It is an exercise that is presented visually to put a persona into a real life situation, and demonstrates how a particular demographic may respond to the product. It allows you to understand what future users will do, use and dislike when using a website.
They should at least outline the WHO, WHAT WHEN, WHERE AND HOW
Scenarios are not predictions or forecasts, but attempts to reflect on or portray the way in which a system is used in the context of daily activity. Once the functions are identified as feasible, the developer can then move onto details in the product.