Worm gears are used when large gear reductions are needed. It is common for worm gears to possess reductions of 20:1, and also up to 300:1 or greater.
Many worm gears have a fascinating property that no other gear arranged has: the worm can certainly turn the gear, but the gear cannot turn the worm. That is because the position on the worm is so 
shallow that when the gear tries to spin it, the friction between the gear and the worm holds the worm set up.
This feature pays to for machines such as for example conveyor systems, where the locking feature can become a brake for the conveyor when the motor isn't turning. One other very interesting usage of worm gears is usually in the Torsen differential, which is used on some Speed reducer high-performance vehicles.