Hi Leslie -

A correction to the good information others have provided:

An ESC (electronic speed controller) will always have a 3-wire cable that plugs into the receiver, whether the ESC contains a BEC (battery eliminator circuit) or not. It's through this cable that the receiver controls the speed of the motor. If the ESC has a BEC, the center wire (usually red) provides power to the receiver. If the ESC does not have a BEC, the center wire does nothing, and you'll provide receiver power by means of a separate battery or an external BEC.

Below is a hookup diagram for the electronics in a typical electric plane where the ESC has a BEC.

Further Explanation:
The ESC (electronic speed controller) connects to the battery, the motor, and the receiver. The ESC serves three main functions.

Controls motor speed
The details of this function differ depending on whether the motor is a brushed or a brushless motor. But in any case, the ESC takes the battery voltage and applies it to the motor in a manner that varies when you move the throttle control on your transmitter.

Battery Eliminator Circuit (BEC)
In electric planes, keeping the weight low is often very important. Since a battery is usually the heaviest component in a plane, it is helpful if you can use just one battery to provide power for both the motor and for the receiver and servos. So most ESCs contain a Battery Eliminator Circuit (BEC). The BEC regulates the battery voltage down to a lower voltage that is appropriate for the receiver and servos. This eliminates the need for a separate, second battery for the receiver and servos. Hence, the term Battery Eliminator Circuit. The ESC connects to the throttle channel on the receiver and sends power to the receiver through that connection.

In larger electric planes where low weight is less important, it is common practice NOT to use the BEC and to instead use a separate battery for the receiver and servos. Obviously, a separate battery for the receiver and servos provides a degree of redundancy, so even if the motor battery fails, you still have power for the control surfaces.

Low Voltage Cutoff (LVC)
Some types of batteries, notably lithium polymer (lipo), can be damaged if discharged below a certain voltage. Most ESCs now include a Low Voltage Cutoff function. As you are flying, if the battery voltage drops below a pre-set level, the LVC reduces or cuts off power to the motor only, while the ESC continues to provide power to the receiver and servos. This gives you a chance to land the plane before the battery is damaged and while there is still enough power to operate the receiver and servos.

- Jeff