Pronation and supination are the motions of twisting your palm to face down and up, respectivly (assuming the lower arm is held horizontally). That’s achieved by having the radius crossing the ulna or parallel. This picture shows the difference nicely. I’ve harped on and on about how Plateosaurus was unable to pronate its hands, and did Matt Bonnan and Phil Senter before me (Bonnan and Senter 2007). Some animals have permanently pronated hands (elephants, for example, and sauropods to a slightly lesser degree). Others have permanently (semi-)supinated hands: theropod dinosaurs, for example.
Humans have an enormous range of motion for pro- and supination. You can see that on the head of the radius: it is perfectly circular (see this photo), thus able to rotate around the long axis of the bone. This range of motion is an adaptation to climbing in trees, among other things, and unsurprisingly we see the same condition is cats that climb a lot (but less so in, e.g., lion).
Today, Dave Hone of Archosaur Musings twittered a great GIF that shows a cat making full use of the shoulder and pro-/supination abilities (edit 20. Feb. 2015: link is down; but similar GIFs here and here). Check it out – it takes a second to understand the camera perspective, but it is really awesome.
Oh, yeah: cats can jump! LOL
Bonnan, M. F. and Senter, P. 2007. Were the basal sauropodomorph dinosaurs Plateosaurus and Massospondylus habitual quadrupeds? In Barrett, P.M. and Batten, D.J. (eds.), Evolution and palaeobiology of early sauropodomorph dinosaurs. Special Papers in Palaeontology 77:139-155.