Frostbite is what happens when body tissues freeze -- ice crystals actually form in the cells of the affected part of the body. If the crystals are large enough, they will burst and kill the cells; therefore, frostbite actually causes damage to skin and soft tissues. The degree of damage is similar to that of burns: first-degree frostbite may cause little more than tingling, a burning feeling, and redness, while second-degree frostbite produces blisters and third-degree (deep) frostbite will damage the soft tissue under the skin. Since it takes time for the freezing to do its damage, frostbite may turn out in the end to be more severe than it first seemed.
Frostbite damage depends on how cold the frozen tissues get. Since blood flow from the center of your body helps keep tissues warm, frostbite occurs most often in parts far from the core -- like fingers, toes, hands, feet, and the tip of your nose. Frostbite is accompanied by vasoconstriction (narrowing of blood vessels, which cuts down on blood flow) in the cold tissue, which keeps the affected tissue from being warmed and makes the cold damage worse. The damage can become even more severe if an affected part is re-warmed and then freezes again. Because of this, frostbitten areas should be rewarmed outside a health-care facility only if there is no chance of them being frozen again on the way to the hospital. Mild, superficial (first- and second-degree) frostbite may respond to blowing warm (not too hot -- you don't want to burn the victim) air over the affected areas, or by putting them in the patient's armpits; the rewarmed parts should be covered with clean, dry dressings after rewarming to protect them from both further cold and other injuries. Deeper frostbite should be treated by putting the affected part in lukewarm water; since deep frostbite can cause permanent damage to tissue, and since people with deep frostbite may also be hypothermic, this should be done in a hospital or an emergency-medicine facility, or at least with your doctor's advice.
Preventing frostbite is mainly a matter of keeping yourself -- especially the more distant parts of your body -- warm. If you are going out in cold weather, protect your hands and feet with appropriate gloves, socks, and boots. Waterproof or water-resistant hand and foot protection is a good idea too, since wet clothes can take heat away from your body and increase the risk of frostbite. In cold, windy weather, you need to protect your face as well -- unlike Rudolph, who can keep his nose warm as well as bright, your nose can freeze up. And be careful of the wind in cold weather: wind-chill factor, or (more properly) wind-chill temperature, refers to the apparent temperature in windy weather. Since high wind causes moisture to evaporate from your skin and clothes, thus lowering the skin temperature, the higher the wind speed, the easier it is for you to be frostbitten.