How compare funtion work in sorting point in Graham Scan Algorithm for Convex Hull?

I am trying to learn Convex hull algorithm.In Convex hull Algo ( Graham Scan ) I am clearly understood the algo but unable to figure out how compare operation work in http://ift.tt/ZpfbqV .
Is it sorting points clockwise or anti-clockwise ? Because after looking code it looks like it is sorting in clockwise order.Please explain compare function code ?I know what orientation function do but only stuck in compare function part.