Frost accumulates when water vapor hits the cold inner walls of
a freezer. The water vapor turns to liquid, and then freezes. This happens in all freezers, but a frost free freezer has three extra parts that prevent build-up of this condensation:
Every six hours or so, the timer turns on the heating coil, which is
wrapped among the freezer coils. The heat melts the ice. When the
temperature sensor detects the temperature rising above 0 degrees
C, it turns off the heating coil.
Frost in your freezer can get to be quite a nuisance. It can get up
to six inches thick if you're not careful, leaving less room for food
and making the freezer less efficient. The good thing about a
frost-free freezer is that it saves you the time and hassle of
manually defrosting.
The bad news is that heating the coils every six hours takes more
energy than would be used by a frosty freezer. Even more energy is
then expended to make the freezer cold again. Aside from the extra
energy being used, this cycle of temperature changes isn't very good for the food being stored in the freezer.
Sources:
www.ebuyguru.com
www.howstuffworks.com