Definition of Use Case Scenarios

Use Cases:

A use case defines a goal-oriented set of interactions between external users and the system under consideration or development. Use cases have become a widespread practice for capturing functional requirements in software design, especially in the object-oriented community where they originated, but their applicability is much wider.

A Use Case Scenario is a description that illustrates, step by step, how a user is intending to use a system, essentially capturing the system behavior from the user's point of view. A use case scenario can include stories, examples, and drawings. Use cases are extremely useful for describing the problem domain in unambiguous terms and for communicating with the potential users of a system.

A useful reference:

Applying Use Cases: A Practical Guide (272pp, 2001)
by Geri Schneider, Jason P. Winters, Ivar Jacobson
Publisher: Pearson Education