Gears improve the speed and agility of your wheels.  They can be used to increase or decrease speed and/or power.

A gear is a rotating machine with cutting teeth, or teeth, which mesh with another toothed part to transmit torque. Two or more gears that work together are called transmission and it can produce a mechanical advantage through a gear ratio and therefore can be considered a simple machine. Geared devices can change the speed, magnitude and direction of an energy source. The most common situation is that a gear to mesh with another team, however, a team may also share non-rotating mesh gear, called frame, resulting in the translation instead of rotation.

In transmissions that offer multiple gear ratios, such as bicycles and cars, the train long term, as in the first place, refers to a gear ratio instead of an actual system. The term used to describe similar devices, even when the transmission ratio is continuous and not discrete, or when the device does not actually contain any gear, as in a continuously variable transmission.