A cogwheel is a wheel with teeth around the edge, so it can catch another such wheel and turn it. You meet it in clocks, old factory machines, and bicycles. People also say gear or gearwheel for the same kind of thing; cogwheel is a little older and more picture-like. Though actually, a cog is strictly one tooth, but speakers often call the whole toothed wheel a cog as well. Cogwheel is the clearer word when you mean the whole wheel, not one tooth. The everyday picture “a small cog in a big machine” — a person with a tiny job in a huge system — almost always uses cog, not cogwheel.