Frostbite is really a type of burn, a cold rather than heat burn and there's different degrees just as there are with thermal burns. First degree will normally just result in numb skin and redness. Second degree will lead to blistering. And third degree results in the black, charred look that will sometimes be bone deep. When it gets this bad, it can become gangrene and may have to be amputated because it's hard to restore a third degree frostbite.
The biggest complication of frostbite is infection. The higher the degree of frostbite, the higher the risk of infection. Obviously, when it reaches third degree and develops into gangrene, infection can develop rapidly and may enter the bloodstream, causing sepsis.
Prevention
The best way to go about frostbite is simply preventing it. If it's windy and cold out, it's good to take regular breaks and get out of the wind. This isn't always possible, but ideal for avoiding frostbite. When going out in the cold, ALWAYS dress appropriately. If it's subzero weather with wind (or even without wind), wear wind proof clothing and make sure to layer well. Wear thick, warm wool socks and consider a second pair of socks (of wicking material). Just make sure it's not too tight as to cut off circulation and make your feet colder. With gloves, you want to make sure to have liners. Normally, you would want a waterproof ski glove with a warm, thinner liner, preferably wool. Make sure to not expose your hands any more than you have to. That's why you want a thinner liner to perform precise tasks without exposing your hands.
Keep your face covered as much as possible. Get a gater or balaclava. The reason people get frostbite to their faces is because they are exposed to the wind and cold. If you suspect severe cold and wind such as in alpine climbing, you may want ski goggles, this will keep the wind off from around your eyes as well as keep the snow and wind out of your eyes. If you wear a stocking cap or hat of some sort, also use the hood on your coat. This will help keep the cold off your neck and blocks some of the wind. A key sign of early frostbite is your skin going numb, if any part becomes numb, make sure to warm it actively to prevent it from turning to frostbite.
Treatment
When on the trail or mountain, you probably won't have too many treatment options. However, there are some ways to treat frostbite. Early frostbite (or frostnip) can be treated by simply boiling snow and holding the warm cup in your hands out of the wind. You can add layers if it's available or use hand warmers. You can, for the most part, actively warm frostnip if you need to.
When it progresses to second or third degree frostbite, DO NOT actively warm. Doing so may cause farther harm. By actively warming, I mean rubbing your hands together, using warm water, etc. You can put your hands in your coat or pockets to help reward them SLOWLY. Warming too fast may cause further problems. If it starts to blister (or worse), wrap it in clean sterile dressing such as gauze. This will help to keep infection down. If it does blister, do your best not to pop the blisters. A popped blister is more susceptible to infection than a closed one (not to mention painful ). You probably don't carry antibiotics with you, so you need to get somewhere as quickly as possible for treatment. I could tell you that as soon as you get frostbite, get home and if it's bad, get medical attention. However, I'm the kind, as with most people, who will finish a trek with frostbite. Just make sure to keep the frostbite clean and change the dressings if you need to. Although, if it progresses to third degree frostbite, you need medical attention as soon as possible.
Frostbite is no laughing matter and needs to be taken seriously. Even if your not climbing Everest and may only be hiking a small park trail, you can still get frostbite. Take precautionary measures to avoid it and your trek will be much more enjoyable.
