Best explaination of what PWM outputs are, and how they work is in the Basic Stamp Manual (on pages 249 to 252 in the pdf version and 247 to 250 in the tangible version)
http://parallaxinc.com/downloads/Doc...ual%20v2.0.pdf
Feel free to e-mail or AOL IM me if you'd like me to explain how the speed controllers take the 5V PWM signals and power 12V motors. (I can explain it, I just don't have the energy at the moment to put it all in this post
)