Shivering is one of the methods that the human body uses to warm itself. It is a neurological reaction, that the body executes when it gets too cold. Joggers are familiar with the concept of moving to stay warm; they run in the coldest of weather and manage to stay warm. Basic physics dictate that energy taken from a storage source (like our fat) and changed to another form of energy (your body movements), results in yet another form of energy - heat. So when your muscles start moving back and fourth rapidly, they make heat, which helps warm the body in the cold. Some people have a different tolerance for cold, and in fact those who shiver easier, can withstand colder temperatures. Specific tolerances can change as we adapt over long term exposure, which is why all those Florida natives look at you funny when you wear your shorts down there in January, they have just adapted to the warm temperatures. Of course there is also the pee shivers.
Shivering is a sign that hypothermia is setting in as well, it's a very early sign, but uncontrollable shivering should be taken seriously. Here's a link to more information on hypothermia it's from Boston but I think they have hypothermia there too:
http://www.bostoncoop.net/balm/hypothermia.html
