A use-case model consists of actors, use cases, and relations among them. Actors represent everything that must exchange information with the system, including what are typically called users. When an actor uses the system, the system performs a use case. A good use case is a sequence of transactions that yields a measurable result of value for an actor. The collection of use cases is the system's complete functionality.
In other words, its a specific way of using the system by performing some part of the functionality. Each use case constitutes a complete course of action initiated by an actor, and it specifies the interaction that takes place between an actor and the system. The collected use cases specify all the existing ways of using the system.
They tie in well with RUP and Iterative approaches.
Use cases and stories are an often more 'friendly' way to extract requirements from users. Instead of perhaps throwing them a requirements spreadsheet and hoping they fill it in, you can almost just sit and get them to tell you the story of how they'll use a system.
the rest are all in the Chat or General (where things do not count) - Admin #1. Mods are cute <img border=0 title= alt=[Smile] src=smile.gif /]