A stepper motor is a kind of electric motor with an internal rotor containing permanent magnets and a set of electro-magnet coils around the rotor which are switched electronically. Stepper motors "cog" to a limited number of positions, but with a microstepping controller, which partially activate adjacent coils so that the rotor rests between positions, stepper motors can rotate more smoothly.

Advantages: Stepper motors will try to lock to the position of the coil that is being driven by the controller so the position of the motor can be known without any actual position sensing. Although it is possible for the motor to be turned out of phase with the coil excitation, which is know as skipping or missing steps, in most cases the reduced cost of the system is justified. Also, stepper motors are "brushless" unlike most other motors and so generate less EMI or Noise.
Disadvantages include lower power efficiency, higher cost per unit, and the need for a more complex drive circuit aka controller.
Control Signals: The stepper motor controller typically accepts two inputs from an external source: Step and Direction. The Direction signal set the direction of rotation and each pulse on the Step signal causes the controller to move the motor one step in that direction. The controller translates these signals into different patterns of current flow in the coils, which result in the moment of the motor. These signals are sometimes called Clock and CW/CCW, but this is still just Step and Direction.
Size: Type 23, NEMA 23 or any other like that refers to the mounting arrangement only (mounting hole pattern and size, diameter of shaft) Also: Table of NEMA sizes showing shaft, mounting hole positions, distance, etc...
Application: Stepper motors excel at driving loads needing up to about 100 Watts of power to move at less than a few hundred RPM with accuracy of 1/200th of a revolution. Lighter loads can be driven faster or with more accuracy. Stepper motors are commonly used to convert milling machines to CNC control.