The margin is the area around the outside of the box. This places space around the element that is transparent - in other words, it's the same color as the background.
The border is a line around the box. The default is for it to be 0 pixels wide, and thus not visible.
The padding is space within the box itself. This is also transparent, but to the background color of the box object itself.
The width is the width of the object, not including all borders, padding, and margins. Note that some browsers interpret this incorrectly, and include the width of margins, borders, and padding in the width of the object, so be sure to test your CSS in various browsers.