Walk into any hotel room, and you will likely encounter a packaged terminal air conditioner, otherwise known as a "PTAC Unit".  The convenience of these units is that they help to keep each room individually at a stable temperature, no matter what the weather outside is like.  They help to reduce the overall energy bills by only heating or cooling single space rooms, as opposed to an entire building at once.

In reality, mankind has always sought out ways to get relief from temperatures of either extreme.  In ancient Egypt, homeowners took reeds, moistened them with water and hung them in their windows in order to help cool the home.  The Emperor Xuanzong of China had a hall built in his palace that was cooled by moving water that turned fans.  Innovation has been the key towards improving the ability to heat or cool ourselves, but major improvements weren't made until advancements in chemistry during the 19^th century helped lay the groundwork for modern conditioning as we know it.

Electromechanical Cooling

The first modern air conditioning unit was invented in 1902, by Willis Carrier.  His knowledge of heating objects with steam helped him to reverse-engineer the process; he ended up sending air over water-cooled coils, which in turn cooled the air and controlled the humidity. He eventually started his own company to produce the air conditioning units. Evaporative cooling, which is the process of water evaporating into the air, was the first "real" air conditioning.  The first residential home to have it installed was in the year 1933.

From that point on, air conditioning units evolved into the machines that we use on an everyday basis.  Hotel chains and hospitals have enjoyed the benefits of PTAC units, as well as the savings from not having to cool or heat entire buildings at once.  Air conditioning continues to play an important role in our everyday lives, and it doesn't look like it is going to go away.