Both air purifiers and ionizers do the same thing: clean the air. Air purifiers clean the air through a mechanical process, while air ionizers use electricity to trap harmful particles in the air.
Air Ionizers operate somewhat differently than air filters. What they do is run the air through a field of ions, which are generated by an electric field. The ions in this field attach to dust particles in the air, causing them to be attracted to a charged collector plate in the ionizer.