A typical stepper motor consists of a permanently magnetized rotor shaft shaped with radial teeth that rotate inside a stator that also contains teeth. Depending on how the stator's teeth are energized, the rotor will align itself in a particular orientation. The stator has four windings that energize various teeth. To drive a stepper, switch the current from one stator winding to the next. When adjacent windings are ON, the rotor positions itself between the two windings.
Steppers move smoothly and are more resistant to resonance effects when halfstepping.
Shaft oscillation occurs when the rotor snaps to the next winding during full stepping. The shaft will first overshoot, then undershoot, continuing a decaying oscillation. If the load on the shaft happens to have a harmonic period that matches the rotor's oscillation, a resonance develops between the motor and the load. This can destroy the stepper's ability to rotate at certain rates. A much bigger improvement in rotor smoothness occurs when microstepping.