Humidifiers put moisture into the air. Because cold air holds less moisture than warm air, when outdoor temperatures drop, inside humidity falls as well. A humidifier is basically a water tank containing a drum or a wheel with an attached water-retaining material. As the drum rotates, it picks up water. A fan, blowing through the drum, sends moisture-laden air into a room.
Some homes have central humidifying systems.
A dehumidifier is similar to a refrigerator. It contains a small refrigeration system with a compressor. A fan pulls in air, which passes over cooling coils, causing water vapor to condense. The condensed water drips down into a pan, which must be emptied regularly. Some units contain a humidistat that turns off the compressor when a desired level of humidity is reached.