Requirements elicitation and representation is always difficult, especially when the results need to be understood by two potentially diverse populations: the stakeholders (users, management, customers, ...) and the architects, designers and developers of the system. Use cases, emphasizing scenarios, are a great middle ground for representing large hunks of the requirements in a style that is both understandable to the stakeholders and specific enough for the technical community charged with architecting, designing and implementing them.
Use cases were introduced by Ivar Jacobson in his book, Object-Oriented Software Engineering: A Use Case Driven Approach, ISBN:0201544350. However, if you are not ready to purchase the book for $60, a great free resource on Use Cases is Rebecca Wirfs-Brock's OOPSLA 2002 tutorial, The Art of Writing Use Cases. You can download a copy of it here. Wirfs-Brock's site is an excellent resource for many OO requirements, design and architecture topics. Definitely worth browsing.
If any one has had experience using Use Cases or knows of further resources, I would appreciate it if you would add a comment. Later.