What is the difference between a cool mist humidifier and a warm mist humidifier?

A cool mist humidifier agitates the water using ultrasonic vibrations or physical force (as in an impeller type) to get some of it to enter the air and become a "cool mist" of water droplets. Therefore, any minerals or germs that are in the water will also get dispersed along with the mist, and a cool mist humidifier must be disinfected regularly to prevent germs from growing in it and spreading to your home. The minerals can form a white dust that you'll find on surfaces when using this type of humidifier.

A warm mist humidifier boils the water, turning it to steam. The steam is then cooled somewhat before it comes out of the humidifier, so it's warm instead of burning hot. Warm mist humidifiers are much less likely to spread germs or minerals, because the minerals don't boil (so they stay in the water tank), and the germs are killed by the heat that boils the water.