A worm wheel gear (which is similar in appearance to a normal gear or spur gear) meshes with a worm gear (a gear in the form of a screw) in a gear arrangement referred to as a worm drive. This setup allows you to either greatly reduce speed or greatly increase torque.
When rotational power is applied to the worm, say, using a motor, it rotates against the wheel and transfers torque to the worm wheel gear. The system is designed so that the worm can easily turn the worm wheel gear, but the gear cannot transmit motion back to the worm. When the gear tries to turn the worm, friction between the two elements prevents the worm from turning.
Worm wheel gears can be found in domestic devices as well as in heavy machinery. They are useful for machines such as conveyor systems, tuning instruments, lifts and elevators, gates and in torsen differentials used in high-performance vehicles and trucks.