Jesse James Garrett does an excellent job of illustrating the layer, the process, the essence of a user-centred design.
Print this off and put it on your wall. Reflect upon what happened when you last were involved in a website from scratch and which elements were overlooked. I have and I do.
Depressing and enlightening in equal measure.
The elements of user experience by Jesse James Garrett - PDF download
This was the foundation for Jesse's subsequent book:
The Elements of User Experience: User-Centered Design for the Web - book by Jesse James Garrett