Fixed Gear bikes are the purest form of cycling. Traditionally these bikes are raced on a velodrome and have a steep frame geometry, narrow forks, a single-speed gear with no freewheel and no brakes. But fixed gears, also known as '''track bikes''', can be ridden on city streets, as bike messengers have known for years and years. There's no coasting on these bikes (hence, the "fixed-gear" aspect), so track bikes reduce cycling to its simplest forms while riders develop an outstanding feel for the road, a more efficient pedaling cycle and a real sense of control over the bike.
Like mentioned above, fixed gears have no freewheel; rather they generally use a 13-22 tooth sprocket, also known as a cog, on the rear hub that's held in place with a lockring. There's three ways to get rolling on a fixed gear bike: buy a preassembled track bike like the Bianchi Pista (very affordable at around $500 and a good start); get yourself some track hubs for a road bike conversion or buy the components and a track frame and build one up yourself.
Track Hubs are the single most important component in fixed gear bikes. They come in high flange or low flange models in varying spoke numbers (usually 28-36 spoke) and you can buy them either as sets or individually. Track hubs can also come as double-sided, flip-flop styles: this simply means one side is threaded for a track cog and lockring, and the other threaded for a single-speed freewheel. Surly, '''I.R.O.''' and Suzue make affordable, good-quality basic track hubs. Suzue's Pro Max hubs are a little higher quality. Traditional track bikes use 130mm spacing or less at the rear, while the spacing on road bikes is slightly wider. Axles on these hubs should be able to accommodate any frame size, but be sure to check. Also quick-release axles aren't recommended, especially for road bike conversions (for reasons I'll explain below) and because they're tempting for a mean person to steal an expensive back wheel. A standard 700c size wheel is fine.
Next up are the cranks. Track bikes will typically use shorter cranks, around 165mm. Since you can't coast around corners, there's a chance for '''pedal strike''', which happens when your pedal hits the street and could cause you to fall. Shimano and Sugino make some nice cranks that aren't too expensive, while, again, higher-end cranks like Shimano's Dura-Ace are available too. Use a single chainring on these cranks, though if you're making a budget conversion you can keep the other chainrings on the crank -- it just looks weird. There's also track pedals, which are small and lightweight and made to use with toe clips. Metal toe clips are lightweight and look nice, but tend to break easier than plastic ones. Some fixie riders use clipless pedals and mountain bike "shoes" (which are more durable and easier to walk in than road bike shoes).
More experienced fixie riders will use an odd-even cog/chainring combination. It's not critical which one is even and which is odd -- the idea is that through controlled skid stops, the rear tire will skid on three different places on the tread, rather than wear one patch down quickly on an odd/odd, even/even setup, and thus keeping your tire alive longer.
Since track bikes are typically built for velodrome racing, there's some aspects that might make a traditional track frame a little less friendly for road use. First off, track bike frames are built with very steep geometry, usually with a tall seat post and a sharp, acute stem. The front forks are generally narrow and the frames aren't drilled for brakes, braze-ons or cables. The frames are very sleek, minimal and built to be quick, so buying a frame that's slightly smaller than what you're used to isn't a bad idea. You definitely don't want the Cadillac of track bikes.
Another difference are the rear dropouts: track bike frames use rear-opening fork ends, like BMX bikes, while road bikes use a short, front-loading vertical, or a longer horizontal, drop out. The rear-opening fork ends make it easier to maintain chain tension, and since there is so much pressure on the rear wheel on these bikes, it's common for the wheel to slip a little over time. With these fork ends, it's not possible for the wheel to slip off the frame if the axle nuts aren't tightened fully or the nuts slide due to an emergency stop. That's why it's important not to use quick-release axles with road bike conversions: if the wheel pops off either in emergency or while riding, you're in trouble.
While experienced riders will stop by using a controlled skid, it's a good idea to have a front brake handy. A rear brake is unnecessary since you have complete control over the back wheel with your legs. Though traditional track frames aren't drilled for brakes, there are some frames that are modified for road use like the Surly Steamroller, '''Gunnar Street Dog''' and the Bianchi Pista.
Chain width doesn't matter much with track bikes, but most use the narrower 3/32" size, which some consider lighter. Both 3/32" and 1/8" chains are about the same strength (for an interesting article about chains click here). Since the rear wheel gets so much use, look for a durable tire like the Specialized Armadillo or the Continental Gatorskin.