Hypothermia occurs when you have abnormally low body temperature. Normal body temperature is around 37°C (98.6°F), as measured by mouth. Hypothermia is defined as a drop in body temperature below 35°C (95°F). Young babies (because they have a greater surface area to body weight ratio) and seniors are the most vulnerable to developing hypothermia.
When the body becomes very cold, all physiological systems begin to slow down - eventually to the point that threatens survival. In some rare cases, though, hypothermia can save lives. There are cases where children who have fallen into freezing water were successfully revived even when there was no heartbeat. Their below-normal temperature slowed down their brain function and caused their bodies to use less oxygen.
Frostbite happens when a part of the body actually freezes. Mild frostbite doesn't leave lasting damage, but severe frostbite does cause permanent damage and may even require amputation of a body part.
Smokers and people with diabetes and other circulatory problems have a higher risk of developing frostbite.