The Nature of Genius

What does it mean for one to be a genius?  Is genius just the upper end of the really smart continuum? Is there a difference between genius and really, really, really, smart?  Is it a difference of kind or degree?  Is genius a matter of smart plus something?  creativity?  insight?  an ability to see things differently?  Can you be a genius, but not be smart at all?  Is smart a propensity and genius a judgment of results?  Can you be a genius, but not have done anything with it?  Is there a difference between being a genius and a prodigy other than age?