Feed-back and Control: Sensors
The Arduino uses a sensor (red board) to sense how fast the motor is turning and provide control of the speed of the motor. There are lots of sensors needed in Robotics and in your car. In your car, there are hundreds of sensors and micro-controllers. The micro-controller (think single use computer) is the heart of everything.
The human body has lots of sensors; eyes, ears, touch, nose, tongue, to name a few. Artificial Intelligence (AI) tries to mimic the human body using sensors and computers, merging everything into a mobile unit (Robotics), this takes lots of micro-controllers and sensors.
There are several places to buy micro-controllers sensors. Digi-Key is one of them.