Freezer burn is a condition that occurs when frozen food has been damaged by dehydration and oxidation, due to air reaching the food. It is generally induced by substandard (non-airtight) packaging.
Cause and effects 
Freezer burn is not a food safety risk. It appears as grayish-brown leathery spots on frozen food, and occurs when air reaches the food's surface and dries out the product. This can happen any time food is not securely wrapped in air-tight packaging. Colour changes result from chemical changes in the food's pigment. Although undesirable, freezer burn does not make the food unsafe. It merely causes dry spots in foods.
The condition is primarily caused by sublimation. Water evaporates at all temperatures, even from the surface of solid ice. If the air adjacent to ice is cold enough (so the ice won't melt into liquid water) and the air is dry enough, water molecules go directly from solid phase (ice) to gas phase (vapour) without ever going through a liquid phase. When the constantly vibrating water molecules in the meats and vegetables stored in the freezer migrate to the surface, crystals of ice outside of the solid food are formed and some water molecules escape into the air (by sublimation). The meat parts now deprived of moisture become dry and shrivelled and look burnt. In meats, air can cause fats to oxidize.
This process occurs even if the package has never been opened, due to the tendency for all molecules, especially water, to escape solids via vapour pressure. Fluctuations in temperature within a freezer also contribute to the onset of freezer burn because such fluctuations set up temperature gradients within the solid food and air in the freezer, which create additional impetus for water molecules to move from their original positions.
It is possible to slow down freezer burn by filling plastic containers with water (leaving room for expansion) and leaving them open in the freezer to help maintain humidity. Proper packaging can also help delay freezer burn because small, air-tight packaging allows local homeostasis of humidity, and, to a lesser degree, temperature, although current available packaging cannot do this perfectly.
Meats and vegetables stored in a manual-defrost freezer will last longer than those stored in automatic-defrost freezers. That is because the temperature of a manual defrost freezer remains closer to 0 °F/-18 °C while the temperature of automatic defrost freezers fluctuates, and because automatic-defrost freezers have drier air, thus the rate of sublimation increases. Food with freezer burn, though dried and wrinkled, is safe to eat. However, food afflicted with freezer burn may have an unpleasant flavour. In most cases, it is sufficient to remove the parts affected by freezer burn.
See also 
- "Whole body freezer burn", a related term used in cryonics
- Ice crystals
- Vacuum packing
- Freeze drying
- What is "freezer burn?"; Everyday Mysteries: Fun Science Facts from the Library of Congress)
- Does "freezer burn" make food unsafe?, USFDA.