Metals such as aluminum naturally form a passivating oxide layer which provides moderate protection against corrosion. The layer is strongly adherent to the metal surface, and it will regrow quickly if scratched off.  In conventional anodizing, this layer of oxide is grown on the surface of the metal by the application of electrical potential, while the part is immersed in an acidic electrolyte.  
In plasma electrolytic oxidation, higher potentials are applied.  For example, in the plasma electrolytic oxidation of aluminum, at least 200 V must be applied.  This locally exceeds the dielectric breakdown potential of the growing oxide film, and discharges occur.  These discharges result in localized plasma reactions, with conditions of high temperature and pressure which modify the growing oxide.  Processes include melting, melt-flow, re-solidification, sintering and densification of the growing oxide.  One of the most significant effects, is that the oxide is partially converted from amorphous alumina into crystalline forms such as corundum (α-AlO) which is much harder.  As a result, mechanical properties such as wear resistance and toughness are enhanced.