Basic Principles of Refrigeration

What is cold?

In terms of energy or physical forces, there is no such thing as cold. Cold is just a word to describe the absence of heat when compared to something else. If we say or observe that one object is colder than another, it really means that the object contains less heat.
There is however, such a thing as heat. Heat is a physical force, a form of energy and it can be transferred from one object to another. Terms such as cold and warm are just words used to describe the relative level of heat.
Therefore, your refrigerator doesn't magically make your food cold, it merely transfers heat from your food to the outside of the cabinet.

Heat transfer

If you were to take two metal cubes, one hot and one cold ( relative to the other), and place them beside each other, they would both soon become the same temperature.

What causes this to happen?

It is a basic law of physics, as sure as gravity, but dealing with how heat transfers from one object or substance to another:

Heat always flows from the warmer object to the colder one.

So back to our refrigerator, the hermetic system supplies a cold surface of about -20* F (-28.8* C) inside an insulated cabinet. The law of physics actually does all the work. The heat that's in your food that you place inside your refrigerator cabinet, eventually gets absorbed through the colder surfaces of the freezer coils, otherwise called the evaporator, and passes into the circulating liquid refrigerant inside the pipes. This transfer takes place mostly through the air inside the cabinet. What happens is the heat in the food transfers to the cool air that surrounds it. Then the air either rises by natural convection or is forced by a fan across the cold surface of the freezer box or evaporator fins in the freezer section. Here the air gives off the heat it has absorbed from the food, then it returns to do more of the same.
The frozen food section of the refrigerator is the area where most of the action takes place. The rest of the refrigerator, where it is not desirable to let the temperature get below freezing, is insulated from the freezer section.
To clear up another common misconception, the freezer and refrigerator sections do not work independently of each other, they are both part of the same system.

Defrosting

One byproduct effect that takes place during the process of air transferring heat to the cold evaporator surface, is that the moisture that is naturally in the air freezes and turns to frost. This frost gets left behind on the surfaces of the evaporator. If allowed to build up for too long, the frost acts as an insulator, and inhibits the ability of the evaporator to absorb any more heat. This effect is undesirable and the frost must eventually be somehow removed. One thing for sure is that defrosting the evaporator does have to take place on all refrigerators in some way or another, no matter what type of fridge you have, or else the fridge eventually stops working. Whether this task is accomplished manually by you, or automatically depends on your type of refrigerator.
This then is the general principal of operation used by all domestic refrigerators to make the inside of the cabinet colder then the outside