Margin is the space around elements.
Padding is the space around the elements contents.
So margin is outside the element.
Padding is inside.
You can use margin when you are placing elements near each other
You can use padding when you are changing how a element interacts with other elements.
(not a good explanation)
Also pixel is the pixels on the screen.
Rem is "Relative to font-size of the root element" (w3schools) https://www.w3schools.com/cssref/css_units.asp
Hope this helps.
