A dehumidifier takes moisture out of the air by condensing it onto a cold surface. Have you ever poured a cold glass of water on a hot, humid summer day? If you have, you have seen the moisture condense on the glass. This is because when air cools, it can't hold its moisture. If you leave the glass on a table long enough, and if the air is humid enough, a big puddle of water will form. The same thing happens in an air conditioner when the moisture in the room condenses on the air conditioner's cold coils. You will notice, if this is a window unit, that water drips out the back of the unit onto the ground. So, a dehumidifier is just an air conditioner that has both its hot and cold coils in the same box. First, a fan draws the room's air over the cold coil of the air conditioner to condense the moisture. Next, the dry air passes through the hot coil to heat it back up to its original temperature.

If you have a room that is air conditioned, it should not need a dehumidifier -- the air conditioner should be doing the dehumidifying for you. But if you don't have an air conditioner, a dehumidifier is at least some help in the heat.