[ What is air-conditioner? ]
The word air-conditioner is the combination of "air" and "conditioner".
It is a machine which raises or lowers the temperature inside a building, controls moisture and distributes clean air to best suit human activities.  In short, it is a mechanical and electric device to adjust the conditions of air.

[ How air-conditioner works ]
When we water the ground in summer, we feel that it gets cool.  Rubbing alcohol over skin also makes us feel cooler.  It is because the evaporation of water or alcohol deprives the area (skin) of heat.  When a liquid more vaporable than water evaporates (vaporize), the evaporation heat -the energy needed for the transformation of a liquid into vapor- lowers the temperature of the area and air is cooled, which makes us feel cooler.  It is very simple.  Air-conditioner lowers temperature with the following 4 mechanical operations.