The WORLD WILDLIFE FUND has lots of great information about Polar Bears. Here is an excerpt from their physical description:
Polar bears are the top predator in the arctic marine ecosystem and the largest terrestrial carnivore. They evolved from brown bears during the Pleistocene, the time period that spanned from 1.8 million to 11,000 years ago.
The polar bear’s coat, covering it completely except for the nose and foot pads, is superbly adapted to Arctic environments. Along with a thick layer of body fat, the water-repellent coat insulates the bear from cold air and water. It also serves as camouflage; in fact, polar bears can sometimes pass as snow drifts. Surprisingly, the fur has no white pigment.
Polar bears are excellent swimmers and can sustain a pace of 10 km/h by using their front paws like oars while their hind legs are held flat like a rudder. Adult males typically measure 6.5 to 8 feet from the tip of the nose to the tip of the tail and weigh 880-1300 lbs. Females are about half this size. The bear’s body and neck are elongated, and the head is narrow and long with small, rounded ears.
They spend much of their time at or near the edge of the pack ice. This is where they are most likely to find food. As the southern edge of the arctic ice cap melts in summer, some bears will follow the retreating ice north to stay close to seals and other prey. Other bears spend their summers on land, living off body fat stored from successful hunting in the spring and winter. When the ice returns in the fall, the bears leave land to resume life on the sea ice. Go to more of WWF’s Polar Bear Overview.
POLAR BEARS INTERNATIONAL is a wildlife educational and outreach site that works only on behalf of Polar Bears.
Polar Bears International is a nonprofit organization dedicated to the worldwide conservation of the polar bear and its habitat through research and education. We provide scientific resources and information on polar bears and their habitat to all interested parties.
Since this is there only focus, the site has plenty of detailed information on Polar Bears and the people who research them. Here you will find up to the minute news about the bears, great video footage from the arctic circle, and information on how you can get involved.
THE NATIONAL WILDLIFE FEDERATION website is loaded with facts about wildlife preservation, climate change, and many endangered species. Their page on Polar Bears is full of detailed information including downloadable pages on Polar Bear facts, videos, quizes and even a polar bear e-card that you can send a friend.
Global warming is causing polar bears to become a threatened species. United States Geological Survey scientists conservatively project that two-thirds of the polar bear population in the world could disappear by 2050, including all of Alaska’s polar bears.
The NATIONAL GEOGRAPHIC SOCIETY which publishes the most popular magazine covering the fate of our world, has an 
excellent website– featuring wildlife, environment and culture around the globe. Here’s an excerpt from their Polar Bear page:
These powerful predators typically prey on seals. In search of this quarry they frequent areas of shifting, cracking ice where seals may surface to breath air. They also stalk ice edges and breathing holes. If the opportunity presents itself, polar bears will also consume carcasses, such as those of dead whales. These Arctic giants are the masters of their environment and have no natural enemies.
Females den by digging into deep snow drifts, which provide protection and insulation from the Arctic elements. They give birth in winter, usually to twins. Young cubs live with their mothers for some 28 months to learn the survival skills of the far north. Females aggressively protect their young, but receive no help from their solitary male mates. In fact, male polar bears may even kill young of their species.
Polar bears are attractive and appealing, but they are powerful predators that do not typically fear humans, which can make them dangerous. Near human settlements, they often acquire a taste for garbage, bringing bears and humans into perilous proximity. Read more National Geographic.