Em

What does EM mean?

Ems are a relative measurement, meaning that the actual point size varies.
One em is equal to the default font size. So suppose a Web browser's default font size is 14 pixels tall. In that case, 1 em would mean 14 pixels tall, 2 ems would be twice that (28 pixels), and 1.5 ems would be 21 pixels.
The advantage of ems is that they allow your Web visitors to control the size of onscreen text. If it's too small, they can increase the base font size. (In Internet Explorer, you make this adjustment by choosing an option from the View Text Size menu [View Make Text Bigger on Safari on the Mac].) Any text measured in ems then changes according to the Web browser's new setting.