Fresh from the clogged tubes of teh intarwubs…
"An overlay is when one web object overlaps another. Overlays are often used to highlight or draw attention to important items on websites to raise conversion rates. This article shows how use CSS positioning to avoid slicing and dicing your overlays and assembling with tables."
"Ironically, absolute positioning is relative. Yes, you read that right. An absolutely positioned element is positioned relative to another element, called the containing block. Here comes the definition of that. Take a few deep breaths and hold on tight to the armrests of your chair. The containing block of an absolutely positioned element is its nearest positioned ancestor, or, if there is no such element, the document's initial containing block."