Let's first talk about why a refrigerator needs to be defrosted, and then we can discuss how a refrigerator defrost system works.
Every time the doors are opened on a refrigerator the hot humid air from the room rushes in. When the doors close that hot humid air is circulated by the evaporator fan over the evaporator. Since there is humidity in the air moisture immediately freezes on to the evaporator and forms frost. If that frost is not removed, the refrigerator air flow is blocked and you will notice an increase in the fresh food section temperature right away. You will also notice that the freezer section is not cold enough, although that will not be as obvious.
The basic components of a typical refrigerator defrost system are, a timer or electronic control, a defrost heater and a defrost thermostat.
The timer or the electronic control allows the compressor to run whenever the cold control usually located in the fresh food section is asking for a cooler temperature. After a specific amount of time the electronic control or timer switches over to defrost. When that happens if the defrost thermostat contacts are closed, the defrost heater will come on. It is not uncommon to see a red glow coming from the freezer when the defrost heater is on. The defrost heater melts frost on the evaporator and the runoff water goes down the drain to a pan located on the base of the refrigerator or sometimes on top of the compressor. This water evaporates back into the room and eventually circulates through the air and sneaks back in the refrigerator.
The defrost thermostat sometimes called the defrost terminator is located on or near the evaporator. Different refrigerator manufacturers and different model refrigerators use different temperature defrost thermostats. Whether it is located on the evaporator, or attached to the wall near the evaporator, a defrost thermostat has a set of contacts in it that open and close based on the temperature. If it has reached sufficient temperature and the contacts are closed, when the timer or electronic control calls for defrost electricity will pass through the thermostat and then through the defrost heater. If however the defrost thermostat is not bound to temperature, the contacts will remain open and the heater will not come on. The reason the defrost thermostat is sometimes referred to as a defrost terminator is because it only allows the defrost thermostat to be on and heating as long as necessary. For example if you were to move your refrigerator and it had been unplugged for a day or so, there is no sense in the defrost heater coming on five minutes after you plug it back in. The defrost thermostat / terminator prevents that from happening.
Do not confuse frost with ice.
If you have frost blocking the airflow of an evaporator and the door was not left open, then you usually have some type of defrost system problem. If you have ice blocking the flow of air on an evaporator most of the time that is an indication that the drain line is restricted. When the defrost heater comes on and melts the frost off of the evaporator and the drain is restricted, the water backs up into the freezer and when the refrigeration system comes back on it freezes. After a few cycles like this evaporator can become blocked with ice.