Humidifiers and vaporizers both release moisture into the air. Humidifiers produce cool mist. Vaporizers produce warm mist.
Both are meant to alter the air in a home or work environment by making it humid and more comfortable to breathe.
Unlike humidifiers and vaporizers, steam inhalers are small and designed to direct humid, warm air directly onto your face or into your respiratory tract.
Most steam inhalers have an internal mechanism that heats water, turning it into steam. Technically, this makes them a type of vaporizer.