A stepper motor is a brushless, synchronous electric motor that converts electrical energy into mechanical shaft rotation i.e. is divided into a discrete number of steps and the motor must be sent a separate pulse for each step. The stepper motor can only take one step at a time and each step is the same size
Stepper motor commonly have a permanent magnet rotor surrounded by a stator. There are also steppers called variable reluctance stepper motors that have four stator windings that are paired with a center-tapped common.You can also buy cnc stepper motor from https://www.automationtechnologiesinc.com/products-page/stepper-motors
This type of stepper motor is commonly referred to as a four-phase or unipolar stepper motor. The center tap allows a change of current direction in each of two coils when a winding is grounded, thereby resulting in a polarity change of the stator. The stepper motor moves in a fixed repeatable increment, which allows one to move it to a precise position.
This repeatable fixed movement is possible as a result of basic magnetic theory where poles of the same polarity repel and opposite poles attract. The direction of rotation is dictated by the stator poles. The stator poles are determined by the current sent through the wire coils. As the direction of the current is changed, the polarity is also changed causing the reverse motion of the rotor.
A steeper motor is widely used device in applications like disk drivers, dot matrix printers, and robotics.The stepper motor is used for position control.