Margin

The margin will add additional space around the current block.  
 Margin (Left & Right) 
 No Margin 
 The block fills it's width (in this case, 100%). 
 
 With Margin 
 The width of the block is reduced by the left/right margin (in this case, 75px). 
 
 Margin (Top & Bottom) 
 No Margin 
 The blocks are touching, with no gaps. 
 
 With Margin 
 There is now a gap between the blocks.