The first question that comes to your mind is what a fixed gear is. This term usually applied to bicycles, which have only one gear. This means that there is only one chain-ring fitted to the cranks, and at the back it has only one sprocket. These two are kept tensioned by a single loop of chain. There is no free-wheel mechanism, and this means that you cannot stop pedaling while the bike is in motion. The forward motion of the bike makes the pedals rotate independently.
Fixed gear provides only one gear ratio. This type of gear is mainly found in bicycles, which is also called a fixed-wheel bicycle, where the free-wheel is not present and runs on only one gear ratio. Fixed gear bicycles cannot coast owing the reason that there is no fixed-wheel mechanism, with the result that the pedals run in the same direction as that of the rear wheel. The rider of the bicycle can only restrict the pedal movement in order to stop the bike without having to apply the brake.
Generally, fixed gear bicycles have one gear ratio. Some are fitted with sprockets on each side of the hub, which provide the use of two gear ratios. There could be fixed gear on each side of the hub, or a fixed gear on one side with a free-wheel gear on the other. In case of a 42 teeth chain-ring there could be 19 teeth on the sprocket on one side, with the other having 17. This makes the gear in the later about 11% to 12% higher than the former.
There are distinct advantages of a fixed gear bicycle. In this type of a bike, either side of the drive drain has one cog each, there are no bearings or pawls in the free-wheel, and there is no deraillurs chain to apply the tension. This brings down the weight of the bike to quite an extent, which provides ease of riding and requiring less force to move forward. The drive system of the bike is direct, owing to the absence of the deraillurs, which would mean that, it provides you with increased pedaling efficiency. Your legs feel the traction between the rear tyre and the road. This makes the bike much ore safe to ride in snow and ice. This is the other great advantage in a fixed gear bike.
In normal bikes, the force on the pedals creates a linear motion to the bike. The pedal in this kind of a bike is connected to the chain-ring, which is the front sprocket. It is connected by cams, which translates the force on the pedals to a torque, and this rotates the chain. Where-as, in a fixed gear bicycle the pedals are directly connected to the rear wheel, and the force that is applied to the pedals makes the rear wheel to rotate. Under the circumstances, a fixed gear bicycle can be ridden backwards too. In this situation, the rider can apply the brake or slow down the bike only by restricting pedal movement.