what
The air conditioner, a form of heat pump for cooling the air inside of a building, has greatly increased the habitability of hot climates for those who can afford the associated energy costs.
how
Air conditioners make use of the basic principle of refrigeration: if you can control the pressure that a substance is under, you can force it to change from a liquid to a vapor and vice versa, with the useful side effect that the equipment you use to do this gets very hot (compressing vapor to liquid) or very cold (evaporating liquid to vapor). Ask thermodynamics for the reasons why. Now separate these two processes: locate the compressor out of doors and the evaporator indoors, but connect them together with plumbing and a pump that circulates a suitable coolant substance from evaporator to compressor and back again. Run this cycle in a continuous loop and you'll have a very hot compressor and a very cold evaporator. Now add an air circulation system to put large amounts of moving air in contact with the evaporator, and you can swiftly cool the air inside of a building to a desired temperature. Connect all of this to a thermostat that turns the system on and off in response to ambient temperature, and you're all set.
There's your air conditioner in simple form. With energy input, the place you care about is made colder, and the wide wide world is made just a tiny bit warmer in exchange. No, you haven't warmed the planet as a whole by separating hot and cold this way (though the generation of electricity to run your air conditioner has contributed, but that's another story).
types
Window box -- small, simple. Installs directly into a window frame and plugs into a nearby electrical outlet, providing cooling for a single room.
Central air -- larger systems. Permanently installed in a separate utility space in order to be less visible in the home or business. Components include a blower to circulate air to many rooms through hidden ducting, as well as fans to increase the efficiency of the heat dissipating radiator outdoors.
coolant
Water, the default terrestrial liquid, is not used as the coolant in air conditioners; there are other substances that can move a lot more heat around, which also do not have the tendency that water has to corrode the plumbing of the air conditioning system if not kept perfectly clean. Many substances called
refrigerants have been developed and a variety are in use. Some have greater environmental consequences than others when it comes to production, leakage, and disposal.
Chlorofluorocarbons (CFCs) in particular have gained notoriety as excellent refrigerants that just happen to wreak havoc on the
Ozone Layer. Particularly as used in automobile air conditioners, which (being mobile) seem to be prone to a lot more leaking and disposal issues than home units.
impact
Air conditioners have greatly changed the living conditions in many areas of hot climate, but are relatively unknown in others due to the great expense of installing and running them. Where I grew up in Sacramento, California one could live without an air conditioner in the summer, but it would be impossible to arrive at work (or anywhere) without being drenched in sweat in the summertime, and home life in the hot season would often consist of sitting in the shade fanning oneself or hoping for a breeze. As a result of modern air conditioning, the human population is far greater in some areas than it might be otherwise.
Air conditioning is also responsible for a large part of the energy spending of many developed areas of the world. In California, for example, unusually hot summers have several times caused an unmeetable demand on the state's energy grid due to millions of families running electricity-hungry air conditioners, resulting in blackouts across parts of the public power system.
energy consumption and change
With the increasingly apparent need to conserve energy, people are learning to decrease their reliance on air conditioners. The thermostat that controls the target temperature of an air conditioner can be set more conservatively to keep the place comfortably cool yet not frigid. Timers can be used to render air systems inactive or in a state of lower power consumption when buildings are uninhabited. There is change, too, at the design level: people are investing more in insulation to decrease the necessity for air conditioning, and planting more trees for shade. A mature tree can lower a homeowner's energy costs by far more than the cost of planting and maintaining the tree.
cooling alternatives
swamp coolers can provide a lower-cost form of cooling in areas with naturally low humidity during the hot season. A swamp cooler saves energy by making use of simple open-air evaporation without the expense of pumping a coolant substance back and forth and compressing/evaporating it. In essence a swamp cooler is simply a wet rug or other wicking object with air being blown over and through it. For cooling of outdoor living areas, very fine water mist can be sprayed into the air for a noticeable cooling effect without overly wetting an area or its inhabitants.
social anecdote
Growing up in a city where air conditioning is taken for granted in the summer, I was accustomed to a lot of very loud noise emanating from buildings all around me in the summertime. Particularly noticeable was the loud hum and rushing air noises emitted by all of the houses in otherwise quiet neighborhoods at night, before midnight, when the air outdoors could still be quite hot even long after sunset. During the hottest times of summer there were times when household air conditioners laboring to achieve even 72°F (22°C) indoors created such a racket that I would get up at night and read instead of sleeping. My next door neighbors had an older model of air conditioner right outside of my bedroom window, and I resented them for being older people who refused to update their equipment when it was clearly twice as loud as anyone else's. In fact, they probably had no idea about the noise problem, and I should have taken a visit to parts of the world that are even hotter with no air conditioning and no central power system to provide cooling. I could have learned a lot about the joy of sleeping in a comfortable hammock outdoors at night with a bug net.
also
See air conditioning for additional perspective.