Pulse Width Modulation
Basically a square wave, scroll down to the last picture:http://www.societyofrobots.com/schematics_h-bridgedes.shtml
You basically turn something off and on really really fast, so instead of being fully on or fully off, you get something in between. This way you can get analog control through pure binary control.
Its like turning a motor on and off really fast by hand so that it isnt running full speed, but it is still moving. That is PWM. Of course it is better with millisecond control, like with a microcontroller, or a 555 timer¬
And of course, the ratio of on to off pulse widths determines the speed of your motor, or brightness of your lightbulb, and so on.¬
Oh and just to not confuse you, servo's are not powered by PWM (at least not by the user). Instead you send PWM just as a control signal that its own internal circuitry interprets. A particular width instead determines the angle the servo is at, or speed at which it rotates if modified, through its own internal feedback control system thingy.