Mark Shuttleworth: The Extra Dimension
Apple calls it Quartz. Microsoft calls it something else but it’s most visible in Aero Glass - the transparent theme in Vista. In the free software world we have Xgl and AIGLX (Ubuntu is going down the AIGLX road). In essence these technologies all do the same thing - they introduce a level of malleability to the once-rigid world of windowing systems.
This is of course a recipe for eye-wateringly poor user interface decisions.