A humidifier is a device that blows cool to lukewarm mist (vapor) into the air to increase humidity (moisture) in a room. A vaporizer is a device that releases a cool to hot mist into the air to help increase humidity or to help with breathing.
These devices tend to be used when a person is sick or when air in the house is dry (low indoor humidity). When a person's skin and mucous membranes are dry, the dryness can aggravate an illness of the head, neck, or chest. Or it can lead to chapped lips, a dry throat, or dry and itchy skin. Mist therapy with one of the devices can help provide relief.
These devices differ in a few ways.
WebMD Medical Reference from Healthwise