A toothed wheel whose teeth engage the links of a chain. 2 : a cylinder with teeth around the circumference at either end that project through perforations in something (such as motion-picture film) to move it through a mechanism (such as a projector)
They're regularly used on bicycles to pull a linked chain to turn the movement of the rider's feet into rotation of the bike's wheels. Sprockets are also used on tracked vehicles such as tanks and farming machinery. They line up with the links of a track and pull them as it rotates, allowing the vehicle to move.