For that purpose you have to use CSS Positioning techniques. Here is one example that shows side by side div blocks (div 1 and div 2) overlapped divs (div 3 and div 4)
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Untitled Document</title>
<style type="text/css">
#main{
position:static;
top:0;
left:0;
}
#one{
position:absolute;
top:1px;
left:0;
width:100px;
height:100px;
border:1px solid red;
background-color:#000;
color:#fff;
}
#two{
position:absolute;
top:1px;
left:105px;
width:100px;
height:100px;
border:1px solid yellow;
background-color:blue;
}
#three{
position:absolute;
top:1px;
left:210px;
width:100px;
height:100px;
border:1px solid green;
background-color:red;
/*z-index:10;*/
}
#four{
position:absolute;
top:20px;
left:240px;
width:100px;
height:100px;
border:1px solid blue;
background-color:gray;
}
</style>
<script type="text/javascript">
</script>
</head>
<body>
<div id="main">
<div id="one">Div 1</div>
<div id="two">Div 2</div>
<div id="three">Div 3</div>
<div id="four">Div 4</div>
</div>
</body>
</html>
Just copy and save the source in a .htm/.html file and open it in your browser. After the first loading try to uncomment the following line associated to #three CSS definition:
/*z-index:10;*/
Save it and reload the page and see the effect.
The following link explains the positioning techniques in a simple manner:
http://www.barelyfitz.com/screencast/html-training/css/positioning/
Hope this helps