No matter how fast you go, there comes a time when you need to stop. Brakes on a bicycle work using friction (the rubbing force between two things that slide past one another while they're touching). Although some bikes now have disc brakes (similar to the ones cars use), with separate brake discs attached to the wheels, many still use traditional caliper-operated rim brakes with shoes. When you press the brake levers, a pair of rubber shoes (sometimes called blocks) clamps onto the metal inner rim of the front and back wheels. As the brake shoes rub tightly against the wheels, they turn your kinetic energy (the energy you have because you're going along) into heat—which has the effect of slowing you down. There's more about this in our main article on brakes.
Rim brakes versus disc brakes
Caliper-operated rim brakes push on the outside edge of the wheel where it's spinning fastest but with least force. That means they need relatively little braking force to slow the wheels (so they can be small and light), though you still have to press hard, and you have to apply that force for longer to bring yourself and your bike to a halt. One big drawback of rim brakes is that they're fully exposed to rain from above and the side and spray from the wheels; if the brake shoes and wheels are wet and muddy, there's considerable lubrication, the friction between the brakes and the wheels could be up to ten times less than in dry conditions (according to David Gordon Wilson's Bicycling Science), and your stopping distance will be much greater.
Disc brakes work closer to the hub, so they need to apply greater braking force, which can stress the forks and spokes, and they're both heavier (which can affect a bike's handling) and mechanically more complex, but they do tend to be more effective in wet weather and muddy conditions.
Browse through online bike forums and you'll find very different opinions about which type of brakes are best for different types of bikes, terrain, and weather conditions. Some people like disc brakes because they make a bike look better; others like rim brakes because they're so simple and straightforward.
