AC (not to be confused with alternating-current), ie air-con or air-conditioning is a very simple idea.
A simple idea, but as important as the air that we breathe.
To be comfortable inside a building, depends on what you are doing and who you are, but some good generalisations are possible.
Four things are key: temperature, humidity, air-flow and quality.
The last of these can be put to one side for the purposes of energy management in a simple sentence - maintain the filters, and know what maximum and minimum flow rates for comfort are (some systems have quality sensors, but I'm not addressing that today as I covered air quality recently in the black art of air quality , and I think there are more opinions than ants at a picnic.
Air conditioning as a phrase is usually reserved for systems that influence both humidity and temperature, whereas simple air treatment often refers to temperature only.
So lets think about replacing air in a building.
If the air that leaves is in a worse state than the air that is coming in, ventilation even without any treatment is beneficial.
So for example if you are trying to cool the building and outside air at 25.5°C (77.9°F) and the air is leaving at 28.3°C (83°F) it is doing cooling for free - so you want as much of it as possible (until extract temperatures fall). So for economy, you should ventilate as a priority over active cooling.
On the other hand, if you are heating, and outside air is below extract temperature - you obviously want move as little air as possible (and recover any heat you can from what is leaving if you have heat wheels, run-around coils or cross-over plates).
The above statements are not strictly true - if you have the ability to detect enthalpy, it can be that cool wet air contains more energy than warmer wetter air. However, if you are using enthalpy control, it is important to be aware of the need to maintain humidity sensors (which are vulnerable to deterioration).
Free cooling is where you vent an unoccupied building at night when electricity may be cheaper and the cooling effect is greater, to mitigate a need to actively cool later. Given good weather forecasts (kWIQly can provide to anywhere on the planet), it can be worth actively cooling at night as cooling plant is more efficient - especially if you have cool storage - an ice store or similar. For monitoring performance good weather histories are essential - refer ventilation rates and degree-days.
Humidity control usually involves drying by cooling to below the dew point - the temperature given air conditions where moisture condenses on a cool surface (think glass of beer :), or humidifying using steam pack or similar.
Evaporative cooling is achieve by letting air evaporate from a pool - this increases relative humidity, so is both most effective and beneficial in dry climates. When viable it is much less energy intense than steam production to humidify or conventional compressor based cooling.
To save money - now is the time to do air conditioning repair maintenance (when AC repairs are cheapest ! )