There is cool article on which some light on difference between Previz and Postviz at digitalcontentproducer.com, do check it out. And here is a great fxguide article on Previz studio Third floor which was born of Lucasfilm Skywalker Ranch and following is a profile video about them by Autodesk.
I think coolest thing is that Virtual Cinematography and GPU rendering is closely linked to Previz to Postviz . Both of these stuff are current hot trends which is driving this dynamic industry and soon it will change the way we work.
Here is amazing talk by Nvidia on GPU technology, do check it out, It's really awesome!!!
On the following video you could really see what they mean by real time rendering.