Understanding the CSS box model is essential for good site layout. The following are examples of how to use the border, padding and margin properties.
A div which is 100px * 100px
The default div
A div which is 100px * 100px with a 25px border
div with border
A div which is 100px * 100px with a 25px border and 100px padding
div with border and padding
A div which is 100px * 100px with a 25px border and 100px padding and 100px margin
div with border, padding and margin