Look at post 29. Scale 4 me posted a picture of a "pulley". What is commonly refered to as a pully would be what you see there. Or if you open the hood of your car and look at the the belsts that drive the power steering, the belt goes around the "pulley". Now, according to Browing MFG, (supplier of power transmissin parts to industry) a "pulley" has a flat friction surface with no grooves and a slight crown to help keep the flat belt centered on the pulley. A "sheave" has a groove for the belt to rid in. The shape of the belt can be in a "V" shape, round or have mulitple V's.

A pulley and a flat belt


A Sheave