Scenario is a specific situation defining a function completed by the user from start to end. Scenarios are helpful in the UX process when you are jotting down a task flow. A task flow is nothing but a textual representation of an action.
For example: Online purchase of a T-Shirt
The scenario here is as mentioned above- Purchase of a T-shirt
The task flow would be a detailed walk through right from login to selection of t-shirt and then check out.