The most important thing when getting in to CSS is to understand the "position" property. Position:static is the default, and is most important. Static divs use the order in which the divs are declared, plus the "float" and "clear" properties, to control where the divs appear on the display. The next thing is to realise that you do not have to use divs all the time. E.g. a single paragraph or image should not be contaned in a div, unless there is an overriding need. Divs are used where multiple elements need to be positioned as a block.
Then, you need to realise that the terms "static", "absolute" and "fixed" are potentially misleading. Fluid sites can be built with any of these settings. It depends on how you use them as to whether the layout is fixed width or fluid.