Fixie

Fixie is the bike with no freewheel, what is a freewheel? is freewheel sprocket block consisting of a single or a set of sprockets mounted on the body that contains an internal ratcheting mechanism and threaded shaft on the hub, so the pedals are always moving. 

freewheel and freehub

 

In Australia these bicycles known as "fixed-wheel" and in the United States as a Fixie. Most riders use a bicycle without using the brake, fixie riders can do the brakes or slow down by going against the pedal crank that rotates.

Many companies sell the bike frame designed specifically for use with fixed-gear hub. A fixed gear or track bike hubs, including a special thread for the lockring is tight in the direction (counter clockwise) opposite compared with that tooth. This ensures that the tooth can not let go when the rider "backpedals" when braking. 

A horizontal dropout on steel frame road bicycle converted to single-speed. Derailleur hanger (below the axle) and hole (above the axle) for mounting a fender or rack, both an integral part of the original frame, now unused.