What is DC Motor?
Answer: Dc Motor is a device used to transfer Electrical Energy into Mechanical Energy. Here is a structure.
What is Gear Motor:
Answer: Gear motor is a DC Motor mounted with a Gearbox(also called gearhead). The gearbox is used to reduce the motor speed while increase the motor's torque.
What is gear ratio?
Answer: the gearbox is used to reduce the motor speed, so it is used a small gear to drive a larger gear in each Gear-Reduction Stage, so there is a reducing ratio, it is each stage ratios multiplied together. Here is a picture to show: