This is where things start to get hairy. The designers of CSS put three layers around blocks of text - headers, paragraphs, blockquotes, etc. The outermost layer is called the margin. The middle layer is called the border, and the innermost layer is called the padding. They did this so you could add a border around the element, and control the space between the border and the content as well as the space between the border and the "outside world". The following stuff refers to these three elements. Also note that Netscape doesn't like making things overlap. So when I say "negative values will make the things overlap", that isn't true for Netscape.
Margins
Margin-top, margin-bottom, margin-left, and margin-right. These take those measurements: in, px, cm, and mm. They increase the size of the margin (the area around the border). Note that you can have a margin of 0 to make the borders back-to-back, or a negative number to make the borders overlap (even into the content).
Borders
Border-top-width, border-left-width, border-bottom-width, and border-right-width. These are the visible areas around a block of text. These take the aforementioned measurement values, and they must be a positive number. These also take the values thin, medium, and thick. If you want all of your borders to be the same width, just use the border-width keyword. That sets all four borders to the given value.
And there's also border-color. It changes the color of the border. If you want all four borders to be the same color, just give it one value. If you want a side to have a different color, you must specify four values. The first value will be the top, the second value is the right, the third is the bottom, and the fourth is the left. Just separate each color with a space, like so: border-color:green red blue purple;. If this isn't used, it'll take on the font color of the element itself.
Lastly, there's border-style. Here are the different styles, with different width and color settings:
border-style:solid
border-style:dotted
border-style:dashed
border-style:double
border-style:groove
border-style:ridge
border-style:inset
border-style:outset
With these special borders, the border has to completely surround the text - you can't have just the left side or just the top visible. But you can use the border-x-width to make a certain part of it fatter than the rest, at least!
Padding
Lastly, we have the white-space between the border and the content, the padding. Padding-left, padding-right, padding-top, padding-bottom. These are the same as the margins, except they're the inside of the border rather than the outside of it.