Track bicycle
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A track bicycle is a type of fixed-gear bicycle specially designed for track cycling in a velodrome. Unlike racing bicycles, used for road cycling, they lack multiple gears and brakes, having a single fixed gear ratio and no freewheeling mechanism. Tires and wheels are narrow, with the tires generally inflated to pressures well beyond those used in road cycling in an effort to minimise the "rolling resistance" caused by friction. (For grass-track events, the tyres would have 'knobs' intended to minimise the potential for skidding.)
Frame design
The design of track frames is usually very specific to its use. Frames intended for sprinting are generally made as light as possible, while those made for general racing or longer events are made as aerodynamic as possible. Frame materials are wide-ranging, including steel (the traditional material for track bikes), aluminium, carbon fibre, or titanium, with carbon fibre being the most common on the professional or elite level. Additionally, the geometry of a track frame differs from that of a road frame, and many frames are designed for specific track events (an omnium frame refers to one that is designed for general track racing). Those attributes common to most track frames include a higher bottom bracket for additional cornering clearance, steeper seat tube for a more forward and aerodynamic position, steeper head tube for more responsive steering, and greater fork rake for more stable steering at high speeds.
Gears
Since track cyclists are unable to switch gears during the course of a race, the choice of gear ratio is very important. At its basic level, a change in gearing is a trade-off of acceleration versus top-end speed. A lower ratio allows quicker acceleration, or 'jump.' This can be crucially important in races where getting a gap on an opponent can make or break the event. On the other hand, a big gear makes it easier to sustain a higher top-end, which is paramount in pursuiting/time trialing, but also important in standard scratch/points races in which the pace will be high.
Ideally, in a mass-start race, the cyclist will find a ratio that balances these two requirements. Without a good jump he risks being gapped by opponents when they jump; without a good high end he'll find himself unable to get around his opponents when the pace stays high.
In order to achieve both top-end speed and jump, track cyclists develop very high leg speed. This allows them to go faster with a smaller gear.