If you look at the Rational definitions (aka RUP), you will find that a Use Case is indeed the graphical representation of the process flow between an actor and a system. It is NOT a graphical representation of requirements.
The Use Case Realization is the document that annotates the steps to realize the flow(s) documented in the use case.
But, those are only the Rational definitions.
I'm curious if the originator of the thread did a google search (or even a search here) before he/she posted?
I often refer to them as useless cases, people are confused of what they, how to produce them and how to use them. Where I am working now they use them as very poor substitute for functional requirements.