Continuous rotation servos are standard hobby RC servos that have been modified to offer open-loop speed control instead of their usual closed-loop position control. The modification effectively turns them into motors with integrated motor drivers. Just throw on a wheel and you have a drive system for your robot that can be controlled using Arduino.
It looks like a servo, it acts like a servo, but it's more than just a servo! We got a factory to custom-make these classic 'standard' sized hobby servos with a twist - the feedback (potentiometer wiper) line is brought out to a fourth white wire. You can read this wire with an analog input such as those on an Arduino, to get the servo's position.
3D Printed Pan Tilt Camera Mount With Servo Motor Package Include: 1x 3D Printed Pan Tilt Bracket 2x 9g Servo Motor (Assambled with the Pan Tilt Bracket) Fully assembled 3D Printed (ABS Filament) strong Pan Tilt Camera Mount With Servo Motor 9g