A sprocket (or reel) it is a toothed wheel or a smaller gear wheel and pinion system, whose teeth are design to hook a chain drive, belt or other item perforated or profiled or other gear.
The sprockets can be used for rotary motion transmission (for example in bicycles and motorcycles, but also in cars and tanks), or for transmitting linear movement in a tape or machine tools.
In motorcycle case, the sprockets are on the wheel and gearbox, and can operate a chain or a belt. In this case, the crown (larger) is mount on the wheel, the pinion (smaller) on gearbox. The choice of the gear is entrust to the gearbox; however, you can replace the crown and pinion system in order to change the final reduction, with it, the acceleration and maximum speed of the vehicle.
To be precise normally we talk about motorcycle transmission pinion gear , referring connected to the gearbox output shaft, whether this is a gear type or continuously variable speed type, which is attached to the rear via a chain, but in addition to the possibility of a chain transmission link between the two organs with a timing belt.