This tutorial by Eugene is an excellent overview of the new node material system. If you are just getting started with nodes or are a bit confused on how it all works then definitely try out this tutorial.
The tutorial is a good read as well. Very well written. Here is a small clip in the beginning:
In order to "get" this tutorial you should already be familiar with general material concepts and how to create materials/textures using the material panel. You should also have a general understanding of the texture coordinate systems available in Blender (e.g. Orco, UV, etc.). Also, when reading this I intend to purposely skip aspects of a node because in later sections you will see the function expanded upon. Each section builds off the previous.
I will begin by saying that the node system does not render the material pane obsolete. Many features and material settings are still only accessible through the material panel (e.g. raymirror). However with the advent of nodes, more complex and fantastic materials can be created since we now have greater control. So lets begin with a normal material (fig 1). Here we have the standard material we have added to a cube mesh. I could, as I have in the past, add color and other settings to this material and it would certainly look nice. But lets say I am just not getting what I am looking for? What if I what to control the creation more tightly or add more complexity? Here is where nodes comes in.
You can find the tutorial here.