Humidifiers contain an interior water reservoir. By running an evaporative belt or drum covered in absorbent padding through the water, the humidifier draws up moisture and then blows it into the air. This creates a gentle mist that increases the humidity levels in a room.
Some humidifiers are classified as "cool mist" units. This does not mean that they cool a room; it just signifies that they do not use heat to boil or evaporate the water to create moisture. While the air a humidifier puts out might feel cool when you put your hand directly into the stream, it is not designed to cool a room.
Humidifiers are most useful in the wintertime, when dry, cold air causes cracked dry skin, irritated respiratory passages and nose bleeds. Running a humidifier will not interfere with your attempts to keep rooms heated comfortably for your family.