A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called cogs), which mesh with another (compatible) toothed part to transmit (convert) torque and speed.
Geared devices can change the speed, torque, and direction of a power source. Gears almost always produce a change in torque through their gear ratio. The teeth on the two meshing gears are the same shape.