Space
En Space
Em Space
En Space
The En is as wide as the lower-case letter n. It is half the width of the Em. I.e. 1 En = 0.5 Em.
The En space for a 12 point font is 6 points wide. (Or 8 CSS pixels).
Em Space
The Em is as wide as the upper-case letter M.
The Em size is equal to the current font size.
The Em space for a 12 point font is 12 points wide. (Or 16 CSS pixels).