Define "Explain". I could write a 15 page report on how electric motors work, how they are made, what they are made out of and more specificly what the Xmod motors do for the Xmods, how and why, the specs of the availible motors, other motors that will work in a Xmod, RPMs, gearing and so on.
What exatly would you like to know???
http://electronics.howstuffworks.com/motor.htm