A buildings HVAC (heating, ventilating, and air conditioning) system can make use of an air-side economizer to save energy in buildings by using cool outside air as a means of cooling the indoor space. When the temperature of the outside air is less than the temperature of the recirculated air, conditioning with the outside air is more energy efficient than conditioning with recirculated air.  When the outside air is both sufficiently cool and sufficiently dry (depending on the climate) the amount of enthalpy in the air is acceptable and no additional conditioning of it is needed; this portion of the air-side economizer control scheme is called free cooling'.
Air-side economizers can reduce HVAC energy costs in cold and temperate climates while also potentially improving indoor air quality, but are most often not appropriate in hot and humid climates.  With the appropriate controls, economizers can be used in climates which experience various weather systems.
When the outside airs dry- and wet-bulb temperatures are low enough, a water-side economizer can use water cooled by a wet cooling tower or a dry cooler (also called a fluid cooler) to cool buildings without operating a chiller.  They are historically known as the strainer cycle', but the water-side economizer is not a true thermodynamic cycle.  Also, instead of passing the cooling tower water through a strainer and then to the cooling coils, which causes fouling, more often a plate-and-frame heat exchanger is inserted between the cooling tower and chilled water loops.
Good controls, and valves or dampers, as well as maintenance, are needed to ensure proper operation of the air- and water-side economizers.