A stepper motor is designed to achieve alignment of its magnetic fields at fixed intervals through its rotation. A controller tells the magnetic fields when to shift and 'take a step'. Typical stepper motors have a resolution of 7.5 per step resulting in 48 discrete angular positions of the motor shaft. Controlling speed of a stepper motor is a trivial operation. Simply step the coils at a set frequency and the speed of rotation becomes fixed.
Stepper motors are excellent for light duty position control.
When a stepper miss-steps it will typically fall backward 3 steps instead of moving ahead one. The reason it falls back is because it is not strong enough to move forward so it aligns itself with an alternate set of coils. A good stepper motor controller is capable of detecting a miss-step and indicating a fault. When a fault occurs the controller shuts down and waits for a user to reset its operation. A fault indicates that the controller is uncertain where the motor shaft is located.