This is a shaft- reversed stepper motor that is frequently used in CNC machines and 3D printers.
The Advantages of step motors are low cost, high reliability, high torque at low speeds and a simple, rugged construction that operates in almost any environment. They are typically digitally controlled as part of an open loop system for use in holding or positioning applications.
This is a Nema 17 stepper motor with a 1.7 x 1.7 inch (43.2 x 43.2 mm) faceplate. The Nema 17 is larger and generally heavier than for example a Nema 14, but this also means it has more room to put a higher torque. However, its size is not an indication of its power.

Specifications：

2 Phases 
Step Angle: 1.8±5% degree/phase
Voltage: 4.2V DC
Current: 1.2A/phase
Resistance: 8.8±10% Ω/phase
Inductance: 6.0mH±20 %( 1kHz 1V rms)
Lead style: AWG26 UL1007
Comes with 70cm 4-Lead Wire
Holding torque: 40N.cm Min
Detent torque: 2.6N.cm Max
insulation resistance：100MΩ Min (DC 500V)
Insulation class: B (130°C)
Rotor torque: 57g.cm 2  
Weight: 280g