CSS unit of measurement 1ex= width of letter "x"
CSS Positioning Concepts
Positioning in 10 easy steps with pictures - very good
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 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