Here's a quick little explanation of motor speed control. Motor equilibrium speed is based on voltage applied. PWM is used to set the voltage. For example, say you had a small DC motor and if you apply 12V it will spin at 1000 rpm. If you applied 6V it will then spin at about 500 rpm. If you have a 12V power supply (say a car battery) you can use PWM to create an average of 6V, or any other voltage across the motor and make it spin slower.
To create 6V, you would use a 50% duty cycle. To do this you could use a replay to switch power on and off to the motor. Usually this type of PWM is done using a frequency of 20 kHz or higher to keep the noise created above the human audible frequencies.