Fixed Gear

A fixed-gear bicycle (or fixed-wheel bicycle, sometimes known in the USA as a fixie) is a bicycle that has no freewheel, meaning it cannot coast — the pedals are always in motion when the bicycle is moving.
The sprocket is screwed or bolted directly onto a fixed hub. When the rear wheel turns, the pedals turn in the same direction. This allows a cyclist to stop without using a brake, by resisting the rotation of the cranks, and also to ride in reverse.

Source: Wikipedia.org