The Box Model is a box that wraps around every HTML element. It consists of margings, borders, padding nad the actual content.
Padding clears an area around the content, whereas, Margin clears and area outside the border.
You set the Width and the Height of the content area
25px controls the padding on the top and bottom of the content, basically the height. The 50px is the width or the padding on the left and right of the content.