These DC motors are equipped with gear package and encoder. It uses 2 phase or 2 channels (quadrature) incremental encoders to detect the acceleration of the motor, path of rotation and the distance it has travelled.
Encoder feedback data allows you 
to regulate your motor acceleration and path efficiently. DC motors with encoder feed back signal usually known as DC servo motors.
DC motors with encoder possess many applications in robots, robot arm, CNC machine and several other applications that require accurate control of swiftness, direction and position. You can control your electric motor using microcontroller or arduino.
DC Motor Specifications
agricultural Chain Voltage: 6:12 VDC
Output Power: 2.2 W
Gear Ratio: 1:75
Rated Speed: 133 RPM
Ranked Torque: 25.4 N.cm
Stall Current: 3A
Encoder Type: Hall impact quadrature encoder 5v (monitor position and path of rotation)
Size: 25 x 54 mm
Weight: 120 g
Shaft Diameter: 4mm