A few years ago there was this breakout trend of converting old road bikes to fixed gear, called them Fixies. Fixed means that so long as the drive wheel rotates the crank must rotate. It allows a certain amount of control that is not afforded by bicycles that freewheel(can coast).
This trend can be linked to bicycle messenger culture that used converted and track bikes as a means of simple, theft resistant and cost effective transportation. When you are going in an out of office buildings the fewer things that can be stolen off your bike the better, and fixed geared bikes at the time were inexpensive on a whole.
But even before that it was common practice for road bikers in the winter, better control and the elements not ruining your gear.
Track bikes live in the velodrome, its a banked oval track. The bikes share a similar dropout to the single speed bikes, but that's where is ends. These bikes are not meant to be ridden on the road, they do not even have mounting points for brakes. They are sleek and sexy and yes people do ride them on the road despite their twitchy and tight nature.
This is one by Chris Bishop, it makes me weak in the knees, another side effect of riding a fixed gear.
Fixed Gear Conversion
This is a Fixed Gear conversion of a road bike, a practice that was common in road riders for a very long time. In fact our beloved road bikes started out fixed.
This is the area of fixed gear bikes that causes the most controversy. Many purists find these an abomination. Largely due to many people chopping derailleur hangers and ripping off other braze ons. This conversion has kept the frame intact, makes it easy put a full groups back on the bike if you decide to later.