Gears are friction wheels with teeth. Gears mesh together and make things turn. Gears are used to transfer motion or power from used to transfer motion or power from one moving the part to another.

A gear or cogwheel is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part as shown in the figure.

Geared devices can change the speed, torque, and direction of a power source. Gears almost always produce a change in torque, creating a mechanical advantage, through their gear ratio, and thus may be considered a simple machine.