Freezing goes back to ice age. Freezing period depends on the quality of food.
I had kept some tomatoes and extra large green leafy bok choy and a bagful of onion stalks from the garden after the fall harvest and kept them frozen. By the time I needed to use up the frozen tomatoes, they were easy to cut frozen but soft and mushy afterwards. However, the tomatoes didn't go to waste because I still cooked them into my tomato sauce for my spaghetti. The frozen bok choy leaves didn't look any good. They came out looking too dark, rot and ugly after defrosting. The onion stalks I had saved in the freezer to be used in making onion soup came out a bit better.
Freezer burn: Freezer burn causes food discoloration esp. on meat. It is said that it is still safe to eat and is not harmful, though however, food inflicted by freezer burn could produce unpleasant flavour. You may trim the discoloured parts caused by freezer burn.
Ice Cream: Freezer burnt ice cream in tubs or carton containers does not taste any good. Scooping away the ice formation around the surface will not save the remaining ice cream. Ice cream dehydrate overtime. To keep any extra ice cream longer, use aluminum foil underneath the lid.
Freezer burn occurs when moisture is sucked away. Again, freezer burn does not contradict to our health. The unpleasantness in appearance makes us not eat it.