Worm gears are often used when large velocity reductions are needed. The decrease ratio depends upon the number of begins of the worm and number of teeth on the worm equipment. But worm gears possess sliding contact which is silent but tends to produce heat and have relatively low transmission performance.
As for the materials for production, in general, worm is made of hard metal while the worm gear is manufactured out of relatively soft steel such as aluminum bronze. This is since the number of tooth on the worm gear is relatively high in comparison to worm with its number of starts being usually 1 to 4, by reducing the worm gear hardness, the friction on the worm teeth is reduced. Another characteristic of worm manufacturing may be the need of specific machine for gear reducing and tooth grinding of worms. The worm equipment, worm wheel china however, may be made out of the hobbing machine used for spur gears. But because of the different tooth shape, it is not 
possible to cut a number of gears simultaneously by stacking the apparatus blanks as can be done with spur gears.
The applications for worm gears include gear boxes, fishing pole reels, guitar string tuning pegs, and where a delicate speed adjustment by utilizing a large speed reduction is needed. While you can rotate the worm equipment by worm, it is usually extremely hard to rotate worm utilizing the worm gear. This is called the self locking feature. The self locking feature cannot continually be assured and a separate method is preferred for true positive reverse prevention.
