The dehumidification process removes excess moisture from the air when the relative humidity is high. Reducing humidity levels is sometimes necessary for health reasons, but it also helps prevent rot, mold, and mildew on interior room surfaces. Air-conditioning systems can accomplish this function as they cool the air in the home. Air conditioners remove moisture from the air as inside air is blown over the much colder cooling coils. Water vapor from the air condenses on the coils the same way moisture condenses on a glass of iced tea on a hot and humid day.
In very humid parts of the country, look for air conditioners that are rated "effective" at removing moisture. The manufacturer's literature will indicate water removal ability in "pints or gallons per hour", which can be used to compare models.
In parts of the country where they exist, basements can create special humidity problems. Some are so humid that even an air conditioner cannot make them comfortable, because it would have to run so long to remove moisture, that it would make the basement very cold. Central air-conditioning systems tend to "equalize" the temperature and humidity throughout the home, so a conditioned basement area will be dehumidified without lowering the temperature to an uncomfortable level.
Basements without central air-conditioning can be made comfortable with portable units called "dehumidifiers". Dehumidifiers contain both a condenser and an evaporator in the same unit. The evaporator removes moisture and lowers the air temperature, while the condenser restores the temperature without adding moisture. Moisture removed from the air must be piped to a drain or emptied regularly. Dehumidifiers are not as efficient as air conditioners, but they are effective under special conditions like basements.