"Flying squirrels do not fly. They launch themselves into the air and glide long distances from tree to tree. They have a membrane known as a patagium that stretches between their front and hind limbs, which serves the same purpose as a hang glider's wings. The tail is flattened, which gives them an even greater gliding surface and aerial control. Northern Flying Squirrels play a critical role in the ecology of Pacific Northwest forests. They are important in the diets of Northern Spotted Owls owl pairs are estimated to consume as many as 500 flying squirrels a year and they help disperse the spores of fungi that aid the forest trees' absorption of nutrients from the soil."

Links:
Mammal Species of the World
Click here for The American Society of Mammalogists species account