PWM

[Tristan Lall]

Basically, its a method of controlling a motor's output without worrying about using a variable resistor (and therefore avoiding the waste of energy involved--it would dissipate it as heat).

Instead of delivering full power all the time and worrying about how to reduce the output later, the Victors (i.e. Innovation First's PWM speed controllers) vary the output's "pulse width" according to some clock signal. They're on for some portion of the time, and then for some proportional amount of time, they're off. On average, the output is equivalent to some percentage, but at any given time, the signal is either off or on. So, for 30% power (average), they would be on for 3/10 of a cycle, and off for 7/10.

Try these links, for more information (I just grabbed a few off a web search).
http://www.powerdesigners.com/InfoWe...s/PWM/pwm.shtm[Edit: DJ Fluck beat me to it...]
http://www.embedded.com/story/OEG20010821S0096

-Tristan Lall