A stepper motor is a brushless DC electric motor which enables precise positional control. It accomplishes this by subdividing the rotation of the motor into discrete steps, which are controlled by the operation of a stepper motor driver.
Stepper motors can be categorized based on many different characteristics. Here are some of the most important ones:
Stepper motor size is fairly standardized by the National Electrical Manufacturers Association, or NEMA. The following is a list of NEMA motor classifications based on size:
These dimesions describe the base of the motor. The height (the dimension along the axis of rotation) can vary depending on the acquired motor.
In general, larger motors can generate more torque, but also use more power.
Note that NEMA standards only describe frame size, and does not indicate that two motors of the same NEMA classification can be swapped (though it does certainly make it more likely).
Motor rotation is discretized into a number of steps. Common steps/revolution standards include:
Motors with more steps per revolution offer more precision, but have lower torque and max RPM.