Nonfunctional requirements describe qualities of a system (what it is) rather than its behaviors (what it does). Scott Ambler inspired much discussion when he recently asserted "Scrum’s product backlog concept works well for simple functional requirements, but… it comes up short for nonfunctional requirements and architectural constraints." in an article on Dr. Dobb’s Portal.
I reported on this for InfoQ, and you can read it here.
Cheers,