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.