CSS Positioning Concepts

CSS unit of measurement 1ex= width of letter "x"

CSS Positioning Concepts
CSS Positioning
Positioning in 10 easy steps with pictures - very good

“position:relative”
The “position:relative” is best used when you want to adjust some element's position slightly. The “relative” means it is relative to the parent tag's rendering box. More here

"position:fixed"
Position is fixed to the browser window. For example "<div style="position:fixed; right:1ex; bottom:1ex">" fixes to the bottom right hand corner of the browser window. More from the original source
When a element is “position:fixed”, it is moved into a layer by itself. Use the "z-index" parameter to specify if it is on top or below the over elements. Again, more here