Scenario is used within personas approach, and appears as a description of what, when and why does the certain persona do to complete a specific task regarding the product. It is a brief story that can be easily created by answering the following questions:
Scenarios can describe the stories and context behind every app or website user. They can be task-based, which means they define only what the users want to achieve (not how) and could be useful for website architecture.
Scenarios can be also elaborated and give more details on a user story. This can help your team to understand how to improve the product and make the user journey easier. Another scenario type is a full scale task that includes all the steps to perform a task. This kind of scenario can remind use cases but the difference is that instead of describing the process from the company point of view, scenario defines it from the user side.
It is important to describe the correct way of how to accomplish the task, probably with some alternative pathways as well. This information will be used by the material observers and it could be helpful to compare how the users were expected to behave and what they did in reality.