There are several different ways you can declare the size of a font in CSS. The units fall into one of two categories - absolute and relative.
Absolute units are fixed. Once they are declared, their size cannot be altered by changing the font size of some other element. Those include
The size of relative units is determined by the size of a parent element. Their size can be altered by changing the sizing of that dependent element. Those include
What units do you normally prefer to use?