Here is a flash animation of a worm gear. A worm drive is a gear arrangement in which a worm (which is a gear in the form of a screw) meshes with a worm gear (which is similar in appearance to a spur gear, and is also called a 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.
Like other gear arrangements, a worm drive can reduce rotational speed or allow higher torque to be transmitted. Worm gear arragement also changes rotation direction 90 degrees.
Assume that disk with red dot attached worm gear in one end. By rotating this disk by clicking on red dot you will be able to worm and worm gear in both direction.