The answer is: "it depends"! Here is
a good article to read.
But to attempt to summarise..in VERY broad terms:
(always) give 1st preference to "static";
consider "float" to achieve desired layout or, for example, to float images within a larger textual-area;
then "relative", but mainly to supply a content for "absolute" and other positioned elements;
avoid "absolute" but, if used, tend to do so in the context of another (relatively) positioned element;
"fixed" is used rarely, but occasionally is used to fix a small element at one side of the browser window.
Specific layouts are mainly achieved using floats, but some layouts can also be achieved with a combination of relative and absolute positioning. Percentage and/or fixed widths on elements are also used to achieve a desired layout and behaviour. cf liquid layouts
People sometimes resort to positioning of elements when they haven't fully explored the use of margins and padding to separate
And avoid DIVITIS
I may have confused rather than helped?? but the links are good