Difference between style = "position:absolute" and style = "position:relative"
Absolute positioning means that the element is taken completely out of the normal flow of the page layout. As far as the rest of the elements on the page are concerned, the absolutely positioned element simply doesn't exist. The element itself is then drawn separately, sort of "on top" of everything else, at the position you specify using the left, right, top and bottom attributes.
Using the position you specify with these attributes, the element is then placed at that position within its last ancestor element which has a position attribute of anything other than static (page elements default to static when no position attribute specified), or the document body (browser viewport) if no such ancestor exists.
For example, if I had this code:
<body>
<div style="position:absolute; left: 20px; top: 20px;"></div>
</body>
...the <div> would be positioned 20px from the top of the browser viewport, and 20px from the left edge of same.
However, if I did something like this:
<div id="outer" style="position:relative">
<div id="inner" style="position:absolute; left: 20px; top: 20px;"></div>
</div>
...then the inner div would be positioned 20px from the top of the outer div, and 20px from the left edge of same, because the outer div isn't positioned with position:static because we've explicitly set it to use position:relative.
Relative positioning, on the other hand, is just like stating no positioning at all, but the left, right, top and bottom attributes "nudge" the element out of their normal layout. The rest of the elements on the page still get laid out as if the element was in its normal spot though.
For example, if I had this code:
<span>Span1</span>
<span>Span2</span>
<span>Span3</span>
...then all three <span> elements would sit next to each other without overlapping.
If I set the second <span> to use relative positioning, like this:
<span>Span1</span>
<span style="position: relative; left: -5px;">Span2</span>
<span>Span3</span>
...then Span2 would overlap the right side of Span1 by 5px. Span1 and Span3 would sit in exactly the same place as they did in the first example, leaving a 5px gap between the right side of Span2 and the left side of Span3.
Hope that clarifies things a bit.
Relative positioning: The element creates its own coordinate axes, at a location offset from the viewport coordinate axis. It is Part of document flow but shifted.
Absolute positioning: An element searches for the nearest available coordinate axes among its parent elements. The element is then positioned by specifying offsets from this coordinate axis. It is removed from document normal flow.
Source
You'll definitely want to check out this positioning article from 'A List Apart'. Helped demystify CSS positioning (which seemed insane to me, prior to this article).