Air conditioning and refrigeration work on the same basic principle. The use of refigerants, substances and blends of substances with low boiling points. The manipulation of these boiling points through the forced change in pressure and temperature.

The changing of pressures is achieved with a pump or compressor and a restrictive metering device, expansion device,  such as a capillary (very small bore tube) or expansion valve. The system consists of a low pressure, low temp side, and a high pressure, high temp side.

Basic principles.
Heat flows from hot to cold
There is a direct relationship between pressure and temperture and evaporation / boiling points of liquids. 

The basic cycle works as follows.

• The compressor compresses the refigerant vapour increasing increasing the heat of the vapour whereby it is discharged into the system at high pressure. 
• The hot vapour is then cooled down within the coils of a condenser to the saturation point for the refrigerant where it changes state and turns into a liquid.
• The condenser is the 'outside unit'
• Liquid still at high pressure flows to a 'metering device'. This metering device is essentially a restriction to the flow of refrigerant.
• As the refrigerant flows through the metering device its pressure drops and thereby so does its temperature.
• Low pressure/ temperature liquid then flows through another coil called the evaporator.
• Heat from the area being cooled, the food compartment of a cold store, or the office space being conditioned is then transferred to the liquid refrigerant in the evaporator coils making it boil. 
• The resulting vapour then flows back at low pressure to the compressor or pump where the cycle starts again.
  

The compressor is lubricated by oil and the low temperature vapour returning to the compressor is used to prevent the compressor from overheating.