And what causes it? How do I know it when I see it?
I believe it is extreme dessication (drying-out). Meat normally has a certain level of moisture, but a freezer is very dry indeed because cold air holds less water vapour.
Try this experiment: put a thin layer of water in a saucer (just barely enough so that it’s only layer of water, not seperated pools). Put that in the freezer and check it two hours later. It will be frozen (duh).
Put it back in the freezer and check it again two weeks later. You’ll see that most of the ice has disappeared.
Freezer burn looks like a dessicated or dried-out patch on food. On frozen chicken breast it sometimes looks like a cross-hatched pattern. Beef tends to look more “burned.” When you defrost meat, any freezer-burned patches wont look or smell “right.” The rest of the meat is fine to use, just cut off the freezer burn.
To protect against freezer burn, seal food tightly against the air of the freezer – including squeezing all of the air out of ziplocs if possible.
And the water that is pulled out of your meat and such is what ends up as frost on the walls of your freezer. You can reduce the amount of freezer burn by wrapping your food in containers that won’t allow the water to pass through and squeezing as much air out of the wrapping as you can.
One technique my father uses for freshly caught fish filets ( no, they weren’t actually caught already filleted 
), is to fill the freezer bag with water, thus encasing the fillet in an ice tomb - It seems to work pretty well for long-term storage of fish ( when we went fishing it was usually ocean rockfishing, which usually generates LOTS of fresh fish ).
Not sure how it would work with other meats, though.
- Tamerlane
This could be due to an auto-defrosting freezer.
It will happen in pretty much any freezer(although possibly faster in a self-defrosting type); evaporation does not cease just because the water is in solid phase.
This effect is put to use by Bolivians; they leave potatoes out on the mountainsides in winter to naturally freeze-dry and they end up with something called Chuno.
My mother grew up on a farm, and said it was easy to dry out laundry in winter. Just put clothes out on the line, they freeze solid, but it doesn’t take long before they’re dry.
The original freeze-drying process.