At the rear they're called sprockets(unless you speak BMX, then they're called drivers), at the front they're called chainrings, or chainwheels(unless you speak BMX, where they might be called sprockets)
A cog is a wheel with projecting teeth. These can turn either another cog or a chain.
velocipede
A cog is a mechanism that allows the transferral of energy from one site to another. A 'cog' is a single tooth on a gear wheel.
cog
bone shaker, hobby horse.
A simplistic answer is: A cog wheel is a circular disc that has teeth round the edge. These teeth usually mesh with the teeth of another cog. If one cog is turned, the other cog moves too. The cogs can be made from metal or plastic.
A bicycle seat is typically called a saddle.
cog town
Another name for a ship is a vessel. What about Boat? Barque? Sloop? Trireme? Galleon? Quinqureme? etc Brig, Brigantine, Fluyt, Schooner, Clipper, Barquentine, Cog, Steamer, Frigate,
Look at your bicycle. As the pedal turns, the chain appearing over the rear cog wheelmoves forward and upwards over the pedal cog, returning by travelling back underneath. Exactly how an escalator works.
That is a 'cog' wheel.
For motorcycles - steel. For bicycles there's an option between steel, aluminium, and even titanium. Although on a bicycle it's usually not called a "main drive cog", so I'm guessing you're referring to a motorcycle.