What does font size actually mean?
Let's take CSS. I have font-size where I can specify a number in pixels, points etc.
So, what does 12px mean? And what does 20pt mean? Is it the maximum width of a character, is it the minimum one, is it the max height?
Will I know for sure that 5 characters of font-size: 10px will have no more than 50px, no less than that, or exactly that width?
Commonly font-size is defined in pixels = px or points = pt
Note: 72 pt equals one inch = 2,54 cm
The resolution of a screen is defined in px. Obviously, the more pixels on a monitor of equal dimensions the smaller each pixel and hence the smaller the font size if font-size is defined in px.
So, What’s the Difference?
It’s easy to understand the difference between font-size units when you see them in action. Generally, 1em = 12pt = 16px = 100%. When using these font-sizes, let’s see what happens when you increase the base font size (using the body CSS selector) from 100% to 120%.
As you can see, both the em and percent units get larger as the base font-size increases, but pixels and points do not. It can be easy to set an absolute size for your text, but it’s much easier on your visitors to use scalable text that can display on any device or any machine. For this reason, the em and percent units are preferred for web document text.
Source: http://kyleschaeffer.com/best-practices/css-font-size-em-vs-px-vs-pt-vs/