WHEN is a bicycle not like other bicycles? To begin with, when it has no brakes, or at least no visible brakes, or possibly just a front brake. That means you can’t ride this bike very well on your first try, and certainly not very gracefully, easily or safely.

The rear cog is bolted directly to the hub, so that whenever the vehicle is in motion, the pedals go around, making coasting impossible. This bike doesn’t have a shift lever or extra sprockets, and the chain is shorter and wider than on traditional bikes.

There are no fenders, and the rear wheels are probably bolted onto the frame to deter theft. You slow down by reversing the pedals, or skidding, or doing a skip stop. And that’s just the beginning of the differences between your run-of-the-mill 10-speed and a track bike, or fixed-gear bike — fixie for short — as it is also known.

Many fixed-gear adherents contend that their bikes are the ultimate and all others are pretenders. And these fixed-gear zealots are a growing presence on the streets of New York. Perceived by some as nuisances, or as troublesome, anarchist Dumpster-diving punks who happen to ride bikes, they are occasionally reviled, but they are also the subject of curiosity and interest. Just as die-hard skateboarders 15 years ago stood on the cusp of providing a new lifestyle, so the fixed-gear bike culture could be the tip of something that nobody can accurately predict but something that is huge.