A worm gear is used when there is a requirement to reduce speed. It’s very common to see worm gears with reductions like 20:1 and as high as 300:1 or even greater depending on the situation.
A worm gear consists of a cylinder with a spiral groove mounted on a shaft, this is generally referred to as the worm shaft and a gear which is normally referred to as the worm wheel. The gear then meshes with the spiral groove on the cylinder and so when the cylinder rotates it causes the gear to rotate as well. So for each complete turn of the worm shaft the gear shaft advances only one tooth of the gear. So a gear with 20 teeth will see the speed reduced by a factor of 20:1.
The worm always drives the worm wheel around it is not reversible so the worm wheel can’t drive the worm to increase the speed. If it’s attempted the system will normally jam or lock.