I answered this very question for someone on Webdesigner Forum so will run through my reply here but in more detail.

Floating is the css method of telling an object to “stick” to the left or right side of the website. Floating is very basic but means it’s easier to keep an eye on what’s happening.

Positioning is more advanced and has many options. You can assign individual margins and even “sticky” an object.

The problem with positioning objects is that it can be highly confusing, as you can end up with objects placed all over the place.
I only use positioning for elements that need to be placed where floating won’t help e.g. overlapping another object.

What do you recommend? Floats or positioning? and why?