Claas Kuhnen put online a serie of 21 videos explaining the basics of Cycles rendering.
If you’re confused about scene and node setup, this very progressive introduction should give you a good start.
It’s clear and didactic and you have some useful tricks also.
Jonathan Williamson explains how to create a blend file used as a node library for materials or compositing functions.
You must create nodes groups, give them a proper name and save all this in a blend file. After you can link this file in your work file or even better, link the file to the default startup file by saving you user preferences just after creating the link.
Bartek Skorupa from Blender Cookie gives a technical explanation of this node.
The video is referencing two other tutorials :