As I am too lazy to come up with my own introduction right now, I’m just going to quote the source. I hope you don’t mind.
Pure CSS-based layouts have come a long way but they still have shortcomings that fail to address certain design goals without compromising the true separation of content and presentation.
In short, the problematic design goals are these:
Total Layout Flexibility
That is, the ability to order columns logically in the source while displaying them in any order desired. For any number of columns.
Equal Height Columns
Or more accurately, equal height columns without having to rely on faux columns.
Vertical placement of elements across grids/columns
Designers face the choice of relying on elements being a particular height, resorting to tables or simply not bothering.
Towards a solution
This article shows how to achieve each of these goals, and then how to combine them, creating what could be called the One True Layout™.
Read all about it at position is everything.