These DC motors are equipped with gear container and encoder. It uses 2 phase or 2 stations (quadrature) incremental encoders to detect the rate of the motor, path of rotation and the distance it has travelled.
Encoder feedback data enables you to regulate your motor rate and direction efficiently. DC motors with encoder feed back signal usually called DC servo motors.
DC motors with encoder have many applications in robots, robot arm, 
CNC machine and many other applications that want agricultural Chain accurate control of quickness, direction and position. You can control your engine using microcontroller or arduino.
DC Motor Specifications
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 direction of rotation)
Size: 25 x 54 mm
Weight: 120 g
Shaft Diameter: 4mm