You can use relative positioning to overlap your elements. However, the space they would normally occupy will still be reserved for the element:
<div style="background-color:#f00;width:200px;height:100px;">
DEFAULT POSITIONED
</div>
<div style="background-color:#0f0;width:200px;height:100px;position:relative;top:-50px;left:50px;">
RELATIVE POSITIONED
</div>
<div style="background-color:#00f;width:200px;height:100px;">
DEFAULT POSITIONED
</div>
In the example above, there will be a block of white space between the two 'DEFAULT POSITIONED' elements. This is caused, because the 'RELATIVE POSITIONED' element still has it's space reserved.
If you use absolute positioning, your elements will not have any space reserved, so your element will actually overlap, without breaking your document:
<div style="background-color:#f00;width:200px;height:100px;">
DEFAULT POSITIONED
</div>
<div style="background-color:#0f0;width:200px;height:100px;position:absolute;top:50px;left:50px;">
ABSOLUTE POSITIONED
</div>
<div style="background-color:#00f;width:200px;height:100px;">
DEFAULT POSITIONED
</div>
Finally, you can control which elements are on top of the others by using z-index:
<div style="z-index:10;background-color:#f00;width:200px;height:100px;">
DEFAULT POSITIONED
</div>
<div style="z-index:5;background-color:#0f0;width:200px;height:100px;position:absolute;top:50px;left:50px;">
ABSOLUTE POSITIONED
</div>
<div style="z-index:0;background-color:#00f;width:200px;height:100px;">
DEFAULT POSITIONED
</div>