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