These DC motors include gear package and encoder. It uses 2 phase or 2 Transmission Chain stations (quadrature) incremental encoders to identify the acceleration of the motor, path of rotation and the distance it has travelled.
Encoder feedback data enables 
you to regulate your motor acceleration and path efficiently. DC motors with encoder feed back signal usually called DC servo motors.
DC motors with encoder have got many applications in robots, robot arm, CNC machine and several other applications that require accurate control of swiftness, direction and placement. You can control your 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
Ranked Torque: 25.4 N.cm
Stall Current: 3A
Encoder Type: Hall effect quadrature encoder 5v (monitor position and path of rotation)
Size: 25 x 54 mm
Weight: 120 g
Shaft Diameter: 4mm