An evaporative cooler, swamp cooler, desert cooler, and a wet air cooler cools air through the simple process of the evaporation of water. Evaporative cooling works by utilizing water vaporization. The temperature of dry air drops significantly through the transition of liquid water to water vapor and water vapor is essentially evaporation. This process can cool air with much less energy than with refrigeration. In extremely dry climates such as the Southwest portion of the United States, evaporative cooling has an added benefit of also moisturizing the air which prevents things from becoming to dry. Evaporative cooling always requires a water source and must constantly have water to operate and cool the air.
An evaporative cooler is really just a large fan that blows air through water soaked pads producing a cooling affect. The fan pulls hot outside air through the pads and into the cooler and blows the cooled air out.