Remember riding a tricycle? A fixed-gear bicycle (or fixed-wheel bicycle, popularly known as a fixie) is like that. It has a direct-drive system between the pedals and the rear wheel. If the pedals move, the rear wheel moves, either forwards or backwards. And if the rear wheel moves, the pedals move. The upshot is that the bike doesn't coast, not even downhill. If you ride 50 miles, you pedal 50 miles, so that the rider cannot stop pedalling. When the rear wheel turns, the pedals turn in the same direction. This allows a cyclist to apply a weak braking force without using a brake, by resisting the rotation of the cranks. It also makes it possible to ride backwards although learning to do so is much more difficult than riding forwards. More recently the 'fixie' has become a popular alternative among mainly urban cyclists, offering the advantages of simplicity compared with the standard multi-geared bicycle.Fixed-gear bikes come in all shapes and sizes. They are put to various uses, such as artistic performance, off-road, utility, cruisers, messengering, touring, and velodrome racing. Fixies can have 3-speed handlebars, mustaches, or MTB bars. They can sport front and rear brakes, fenders, panniers and other components. And a few fixes even have more than one speed. Virtually any bike can be converted into a fixed-geared bike. As a rule, fixed-gear bicycles are single-speed. A derailleur cannot be fitted because the chain cannot have any slack, but hub gearing can, for example a Sturmey-Archer 3-speed fixed hub. Most fixed-gear bicycles only have a front brake, and some have no brakes at all.http://en.wikipedia.org/wiki/Fixed-gear_bicycle