Nitro Fumes

I've many years of experience with nitro but electric has advanced in technology since the last time I had an electric, in the '80's racing 1/12 pan cars with 12 maH NiCads. I really didn't like electrics back then. Nothing but trouble. Nitros were (and still are) quite trouble free. Now there lipo batteries and brushless motors. So as far as the brushless motors are concerned there are "sensor" motors and "unsensored". And of course the ESC's now have to accomadate the sensors. So wht I basicaly want to know is about the sensor motors. What does the sensor do? Is it some kind of anti skid control. Can someone explain? Thanks.