1) The em is the font-size of the current font.
It is a relative measure, so:
1 em equals 1x the height of the current font.
2 em equals 2x the height of the current font.
2) The point is a unit of measure from printed type, and is equal to 1⁄72 of an inch.
Which means the height of a 12 pt font is: 12 * 1⁄72 inch = 1⁄6 inch.
3) To determine the font height on screen, versus the printed page, we need to convert 1⁄6 inch to pixels.
The CSS specification defines a pixel to be 1⁄96 inch.
And 1⁄6 inch * 96 pixels per inch = 16 px.