em
An em is a unit of typography, equal to the currently specified font-size.
p { font-size: 16px } /* 1em = 16px */
html { font-size: 16px }
h1 { font-size: 2em } /* 2em = 16px * 2 = 32px */
rem
rem means root em.
It is a unit of typography equal to the root font-size. This means 1rem is always equal to the font-size defined in html.