Getting your head around the box model for CSS can be a bit of a mind-bending exercise for many people. Just try to explain it to a friend who doesn’t already know much about it, and you’ll see what I mean. Its not that its a difficult concept to grasp, its just very fiddly to articulate.
Thankfully, this entry from Jon at 3nhanced demonstrates the box model in action with some simple diagrams. If you’ve ever had trouble getting to grips with the box model; read this guide and you’ll be kicking yourself afterwards over how simple it actually is!