The difference between a humidifier and a dehumidifier is that one takes moisture out of the air and the other adds moisture to the air. A dehumidifier draws air over a set of cold coils to cause condensation. The condensed water is collected into a container for emptying.
Too much moisture in the air causes mold and foul odors. It also creates a breeding ground for mites and other insects. A humidifier is used to add moisture to the air. This prevents wood from drying out and contracting. It also reduces the occurrence of dry eyes, nasal passages and throats. The correct humidity inside a building for comfort without causing mold to grow is between 30 and 55 percent.