Sprockets are rotating parts with teeth that are used in conjunction with a chain and, almost always, at least one other sprocket to transmit torque. Sprockets and chain can be used to change the speed, torque, or original direction of a motor. In order for sprockets and chain to be compatible with each other they must both 

A sprocket or sprocket-wheel is a profiled wheel with teeth, or cogs, that mesh with a chain, track or other perforated or indented material. The name 'sprocket' applies generally to any wheel upon which radial projections engage a chain passing over it. It is distinguished from a gear in that sprockets are never meshed