Gears are used for transmitting power from one part of a machine to another and in tons of mechanical devices. They do several important jobs, but most important, they provide a gear reduction in motorized equipment. how do they do it? gear want to work will have to be pair of gear wheels each other. gear have teeth cut around the edge to stop or to give them or power to slipping. two different size of gear creates lever.
When you have lever you get torque. This is key because, often, a small motor spinning very fast can provide enough power for a device, but not enough torque. And gear can use this power to move many type of mechanical devise as long as the gear type and size are right. And place more the 2 gears in box and often people will start to call it gear box.
Another thing gears do is adjust the direction of rotation. For instance, in the differential between the rear wheels of your car, the power is transmitted by a shaft that runs down the center of the car, and make gear angle in 90 degrees to apply it to the wheels. Or make 60-degree angle for brush cutter to power the head part in order get blade turning. And or hand tool power tool or some other type machine need to do power transmission always will see them.