This is for a class I teach so if you’re not in the class some may be out of context but hopefully still helpful.
If you don’t understand the BOX MODEL then you should watch http://www.youtube.com/watch?v=AWHAT2Iksk0 first. Ignore the first 20 seconds or so if you are not in my class. Once you understand that then come back to this video.
This video is part 2 of a 3 parter. The whole series is designed to help you learn how the 960 grid system works but the first two, including this one, are applicable to all web developement.
How the Box Model works
How CSS Floating Works
How the 960 Grid System Works