Friction between the chain and the gears needs to be minimized to reduce the energy losses in this part of the transmission. Two chain types are illustrated below. A, is a 'block' chain with solid links connecting the side plates. With this chain type, the blocks

slide over the teeth of the chain-wheel and sprocket and friction losses can be high, even with good lubrication. The lower, B, diagram shows a roller chain with the transverse links between the side plates being able to rotate about a central axis. Friction between the rollers and the gear teeth causes them to rotate rather than slide and the energy losses are reduced. This chain type is used on most modern bikes.

From: Encyclopedia Britannica On Line
http://www.eb.com:180/eb.html