Flying squirrels do not fly, but glide from one perch to another. Their "flight" is made possible by a fold of skin, a membrane which extends from the front to the hind feet. When the legs are outstretched, the skin stretches out tautly to form a large planing surface which enables the squirrel to glide as far as 150 feet, though most glides are between 20 and 30 feet.
For more information, see
http://www.dnr.state.mn.us/mammals/flyingsquirrel.html.