You will find that all RC helicopters come with packaging that states they have an ESC. What is an ESC? It’s actually the electronic speed controller. If your copter comes with a variable speed motor, and it most likely will, then the electronic speed controller is the device that controls the speed.
The electronic speed controller works by turning the signals from your throttle controller into small adjustments that can control the speed of the motor. The type of ESC can vary depending on the type of motor you have. If your motor is the variety with brushes then all that happens is the ESC controls the amount of power that is delivered to your motor thus changing the speed. The power is delivered in pulses (or chopped – excuse the pun) so the longer the pulses the more power the motor receives so the faster it goes.
If you have a brushless motor the speed is controlled by changing the voltage across the windings. There are no brushes to change in these motors so maintenance is reduced. They are now becoming more and more popular because of this.
In summary make sure you use the right ESC for the right type of motor and you won’t go wrong!