em vs rem

em's and rem's are pixel (px) size multipliers mostly for fonts
em's are based on the size of their parent
rem's are based on the size of the document (i.e. html)

I'm parent div set to 20px

I'm the child div set to 2em, i.e 40px.

I'm the child div set to 2rem of the html setting of 30px, i.e 60px.