Difference between em, rem, and px?

Question

What's the difference between em, rem, and px & which one should I use / why?

Answer 1

https://www.w3schools.com/cssref/css_units.asp
https://www.tutorialspoint.com/css/css_measurement_units.htm
http://mdn.beonex.com/en/CSS-2_Quick_Reference/Units.html

Answer 2

I recommend px