difference between a DC motor and gear motor
A DC motor is not the same as a "gear motor" - a "gear motor" may be an AC or DC motor coupled with a gearbox or transmission. A gear motor adds mechanical gears to alter the speed/torque of the motor for an application. Usually such an addition is to reduce speed and increase torque. A DC motor without gears is useful in many applications, for example the simple fan. Contrast that with the motor that operates the turntable in your microwave, a gearbox there reduces the speed so your food isn't thrown against the interior wall.

For more information, check the Wikipedia on DC Motor.