CSS: px, pt, and em

The difference between px, pt and em. - The CSS Weblog - css.weblogsinc.com _: "Em is simply a relative form of measurement. If you set all of the fonts for within the body of your page to be 12px, for instance, by setting an em of 1.5 (i.e. font-size: 1.5em) would make your text be 50% bigger (150% of the current size) than your base font size. So, if you want percentage-based font sizes for all elements on your page, then you can set all but your base font size with em?s. It comes in really handy if you suddenly want all of the text on your site to be slightly larger? all of the other elements will follow the leader."