This css tutorial is from the Mozilla Developer Network, here.

Division Element #1

position: relative;
z-index: 5;

Division Element #2

position: relative;
z-index: 5;

Division Element #4

position: relative;
z-index: 6;

Division Element #3

position: absolute;
z-index: 4;

Division Element #5

position: relative;
z-index: 1;

Division Element #6

position: absolute;
z-index: 3;


For more information on Positioning, see this blog post.

And this tutorial.