Layering Modularity is a fundamental concept of system design. By providing a clearly defined functionality that the world already wants for other reasons, platforms make it much easier to develop new information services. Messerschmitt explains the concept of a layer at some length, but here is a simple version.
Networked services are complex, and so to manage that complexity they are organized in layers.
Each layer provides a distinct and well-defined service. In particular, each layer defines a clear contract that governs its interactions with the service layers upon which it is built, and the service layers that are built upon it in turn. Another example of a service layer is SSL, the Secure Sockets Layer for the secure transmission of sensitive information between clients and servers on the Web. SSL is built on top of TCP and IP, and other, more complex service can be built on top of SSL in turn. All modern networked services are organized in layers.