What causes freezer burn?
How can I tell if meat has freezer burn?
Freezer burn is just the food being dehydrated. Most meat will change color, and it becomes very obvious if you defrost it. It's not unsafe: just yucky.
Leaving foods unwrapped or directly exposed to the air in your freezer/refridgerator will contribute greatly to freezer burn - try to put leftovers in containers or wrap them to ward against freezer burn :)