What is the difference between px, em and ex?

1. Pixels (px) are browser dependent. It is the absolute size that you would see on your screen.
2. Em are sort of like percentages. Ems is referring to the base text size. The value of 1 em means the same thing as a value of 100 percent. But you can also say it in the opposite way: A percentage value is just an em multiplied by 100.
3. Points(pt) are what you would want to use in print media.

em : the font-size of the relevant font
ex : the x-height of the relevant font
px : pixels, relative to the viewing device