V-belt is a belt of V-shaped cross section engaging a V-shaped groove in a pulley for wedging and better traction.  
V-Belts are friction based power or torque transmitters. The power is transmitted from one pulley to the other by means of the friction between the belt and pulley. The rubber used as the base material plays a very vital role in this. This is quite similar to the friction between the Tyre and road in the automobiles that enables the automobiles to move on the road.
The V-Belt is called so because of its cross section. The cross section of a V-Belt is similar to that of the letter ‘V’.