Tech Note.
Stepper motors in simple terms are multiple electromagnets which drive a rotary shaft. The shaft is driven by switching the individual magnet coils in a particular sequence. Reversing the sequence will result in the output shaft changing direction. Depending on the construction and the correct drive circuitry the output shaft will turn a precise amount at a precise speed. The amount the drive shaft turns from a single pulse is determined by the mechanical 'Step Angle' of the motor and is measured in degrees. For example, a stepper motor with a 1.8 degree step will turn the drive shaft one complete revolution every (360 / 1.8) 200 pulses. A motor with a rated step angle of 30 Degrees will turn the drive shaft one complete revolution every (360 / 30) 12 pulses. Equally so, if only half the pulses are applied, the shaft will only turn half a revolution ! Stepper motors are bushless and are very reliable, they are incredibly versatile devices and can be used in a host of control applications were precise positioning is required. The downside of stepper motors is that they do require electronics to function, although often only a single integrated circuit or PIC is required for full control.
For more detailed information see the following links:
Stepper Info1 or Stepper Info2 or Stepper Info3