An RC servo is used for controlling the steering wheels of the car. It almost always comes with the RC car, so you shouldn’t worry about getting one.
It typically expects around 4.8V to 6V input on the power wire (varies by car) and a PWM control signal on the signal wire. Typically, the three wires are colored black-red-white, or brown-red-yellow, where:
The control signal is RC-style PWM, where one pulse is sent 60 times a second, and the width of this pulse controls how left/right the servo turns. When this pulse is:
This is NOT the same kind of PWM that you would use to control the duty cycle of a motor, or the brightness of a LED.
The power for the servo typically comes from the motor ESC, which has a BEC (Battery Eliminator Circuit) built in.
Source: DonkeyCar docs.