There are four basic types of humidifiers:
ULTRASONIC: Works by passing water through a nebulizer (a device that turns liquid into a fine mist). A fan circulates this mist into the room where it evaporates. Some ultrasonic modes allow you to choose between a cool and warm mist output. Ultrasonic models may have either an ionizer stick or built-in nano-silver technology that helps to prevent and control bacterial growth in the air.
COOL MIST: Water is drawn into a filter or wick, which removes mineral deposits and keeps bacteria from getting into the air. A fan then sucks this moisture out and returns it to the air, thus cooling down the room temperature and making it easier to breathe. This model constantly adjusts its return of moisture to the air, making it the most energy-efficient type of humidifier simply because it does not allow for over-humidification. The cool mist usually makes breathing easier for asthma and allergies. If you find it hard to breathe in a sauna, a cool mist would probably be better for you. On the other hand, if a sauna makes it easier for you to breathe, a warm mist would be a better choice.
WARM MIST: Water is boiled to kill the bacteria and the resulting steam is then dispersed back into the air. This can actually warm the air and use more energy, but because of this warm air action, it can also be used as a vaporizer with medication. Some models even have a UV bulb for an added level of protection against germs and bacteria. The humidity should be controlled by a humidistat, so the machine does not over-moisturize the room.
NOTE: A humidistat regulates the humidifier's output in order to maintain the desired level of humidity in the room. When the humidistat recognizes that the desired level is reached, it stops the humidifier and when the humidity drops below the desired level, it will start the process over again. A humidistat is essential on warm mist models and found in some cool mist models.
AIRWASHER: This type of unit combines the effects of an evaporative humidifier with those of an air purifier. Disk stacks rotate in water. As dry air meets the surface area, the water is evaporated and dispersed into the room. Airborne particles are drawn in and pass over the disk stacks. Particles get pulled down into the water and remain inside the unit.
Certain models feature automatic shut-off when the water tank is empty, so it doesn't dispose any unwanted mineral deposits or white dust (that may settle in an empty tank) back into the room. Other models have a refill light so you know when your water tank needs refilling. Obviously you don't want your humidifier to run out of water, since it makes the job of moisturizing the room virtually impossible.
Sizes range from a tabletop size for a single room, to a large household unit that can humidify much of an average size home.