Margin is outside of a box between the border and browser window, while padding is inside the box between content and border.
Box model is CSS's design of all HTML elements, essentially every element is wrapped around a box, and is distinguished between content, padding, border, and margin.
"em" is suprior to "px" when setting font sizes because "em" allows user to change the font size when browsing while "px" sets absolute font sizes.
In this example:
The margins are: