A servo is are awesome for controlling an RC car or robot. Inside each one is a DC brushless motor, some gearing, electronics, and a potentiometer. What does all that mean to you?
Just tell them where to go and they do the rest. "Move to 45 degrees!" and they just do it. No questions asked.
All you have to do is know what 0, 90, and 180 degrees means for each servo. Then you can start building interesting applications.
You tell them where to go by sending a Power Width Modulation (PWM) signal. The Arduino UNO R3, for example, can send 6 PWM signals at once. The MEGA has a crazy number of PWM signals.
The gear sticking out the top is 5.5mm in diameter
Range: 180 degrees
Internal gears are plastic.
These servos can be directly connected to an Arduino and run without rebooting the board. Great for your first experience!