gear v. sprocket v. pinion v. cog

• A gear is a rotating toothed wheel. The teeth of an input gear mesh directly with the teeth of an output gear, and in so doing, alter the direction, speed, or torque of the input motion. Gears have applications in diverse machines, from clocks, to car transmissions, to kitchen appliances.
• A sprocket is a toothed wheel that engages with a chain, track, or other perforated material, but not directly with another sprocket. Sprockets are used on bicycles, tanks, and movie projectors.
• In a gearing system, the pinion is the smallest gear, as in a differential. A pinion may also be a gear that engages with a rack, converting rotational motion into translational motion.
• A cog is a tooth on a gear, but may also be used to mean cogwheel, a synonym for gear.

How Stuff Works on gears