A Work Breakdown Structure is "the architecture of the project plan" according to Royce. It is a decomposition of the project plan into discrete tasks. Royce describes the modern version of a WBS as one that is structured around the process framework, whereas in the conventional process it is broken down using the product design into functional tasks. This early decomposition of the conventional WBS is one of the problems of the conventional process, because it locks in the architecture of the system before any coding is done. The point of the modern process is to allow the architecture to develop during the iterations in order to find the best one -- rarely are you going to find the best architecture before writing a singe line of code.
This is a framework, according to Royce: