What is the difference between px, em and ex?

Question

What is the difference between px, em and ex? And when you define font-size in CSS, do you use px, pt or

Answer 1

what is the difference px,em and ex?

http://www.w3.org/TR/CSS21/syndata.html#length-units describes those and the other length units available in CSS

And when you define font-size in css, do i use px,pt or em?

As a rule of thumb, use percentages on screen and pt for print.