I've set all of the content on this page inside a 10-pixel border (black top and bottom, white sides) with style set to "inset" and padding set to 3 pixels. In addition, the body has a margin of 5-em top and bottom, 10-em right and left.
An "em" is a typographic measurement which corresponds to the point size of a font: an "em" is roughly one letter wide. The name apparently derives from the widest letter in most fonts, "m"; half of an "em" is an "en," named for—you guessed it— the letter "n," which is half as wide as "m." Those dashes in the previous sentence, by the way, are called "em-dashes"; they are not hyphens (the hyphen "-" is an "en-dash").
Enough of this . . .