You want to slow your descent, but not stop. Your first thought may be to apply the brakes lightly all the way down the hill. Friction will heat the brake pad and wheel, possibly causing a tire blowout or ruining the brake pads and leaving you brakeless. On long downhills, brake in short bursts, a few seconds on, a few seconds off. This prevents heat buildup along the wheel.
Coaster brakes are common on single speed bicycles.
Coaster brakes are operated by pedaling back ward. There are no hand levers, and there is no front brake, though some bicycles come with front hand operated brakes and rear coaster brakes. Coaster brakes don't stop a bike as quickly as front and rear hand operated brakes, but they don't lose their stopping power in wet weather as hand brakes do. Gears are simply a way of helping you pedal up hills and on flat terrain with a minimum of effort. Most bikes have from 3 to 18 gears, depending on the model and its in tended use.