The declaration of margin property and its effect on element.

All four sides have a margin of 10 pixels.

margin:10px 20px;
Top and bottom have a margin of 10 pixels.
Right and left have a margin of 20 pixels.

margin:10px 20px 30px;
Top is 10px
Left and right are 20px
Bottom is 30px

margin:10px 20px 30px 40px;
Top is 10px
Right is 20px
Bottom is 30px
Left is 40px