A worm drive is a gear arrangement in which a worm meshes with a worm gear. The two elements are also called the worm screw and worm wheel. The terminology is often confused by imprecise use of the term worm gear to refer to the worm, the worm gear, or the worm drive as a unit.

Meaning you rotate the worm shaft say clockwise, the worm wheel will rotate forward, and vice versa, but you cannot do the opposite meaning, you cannot rotate the worm wheel and make the shaft rotate in one particular direction. Reason: This drive is friction based drive and not positive drive.

A gear is a kind of machine element in which teeth are cut around cylindrical or cone shaped surfaces with equal spacing. By meshing a pair of these elements, they are used to transmit rotations and forces from the driving shaft to the driven shaft.

There are many types of gears such as spur gears, helical gears, bevel gears, worm gears, gear rack, etc. These can be broadly classified by looking at the positions of axes such as parallel shafts, intersecting shafts and non-intersecting shafts.