Pulse Width Modulation is a workhorse in electronics.
Essentially, PWM is a fancy way of saying: turning the power on and off really fast such that it looks like you have a constant stream of a lower amount of power.
A common use of PWM is for the control of motors. Especially in the case of large motors, the inconsistency generated by high frequency stepping is all but wiped out by the inertia of the motor itself.
Neat stuff. This clever clown explains it nicely.