A basic - but often misunderstood - feature of CSS styling is something called the box model, which defines the rectangular space around an element by means of border, margin and padding settings. It's really very simple, but a combination of poor nomenclature and some subtle rendering rules confuse folks who are new to CSS.
To help you get up to speed with the box model, I wrote an article over at The Code Project called CSS Basics: The Box Model, Margin and Padding. Please check it out and, if you find it helpful, vote it up. I hope you learn something useful.