A gear is a toothed wheel that connects or meshes with another gear. It is used to transmit forces in machines that range from something as small as a watch to something as large as the locks on a river system. There are four basic types of gears. A worm gear is used to obtain a large reduction in speed (e.g., hand held food mixer). A bevel gear turns the movement 900 (e.g., can opener).
A spur gear is a standard gear and transfers motion in the same plane (e.g., bicycle).
The fourth type of gear is the rack and pinion gear. It is used to convert rotary motion to linear movement (e.g., focusing an overhead projector)