Painless Functional Specifications - Part 2: What's a Spec?
http://www.joelonsoftware.com/articles/fog0000000035.html

Spolsky, Joel
Joel on Software
2000

Abstract:

When you design a product, inside and out, the most important thing is to nail down the user experience. What are the screens, how do they work, what do they do. Later, you worry about how to get from here to there. There's no use arguing about what programming language to use before you've decided what your product is going to do. In this series, I'm only talking about functional specifications.