how it works

[farishta]

System of bicycle gears consisting of multiple sprockets ,a chain and a mechanism to move the chain to the correct sprocket. (10)

[Mortartube]

Basically, there is a metal adjuster cage that sits over the chain, (stick your thumb and forefinger straight out and iminge the chain running between them). When you adjust the gear lever, this cage moves sideways and physically pushes the chain from one size sprocket to another whilst you are pedalling). More here.

http://www.sheldonbrown.com/derailer-adjustment.html

[andy hughes]

I think, it's 'derailleur' - the gear system most commonly used in bikes of modern manufacturre.

The name was famously mispronounced between the members of Cream - giving rise to the title of their excellent album Disraeli Gears.

[fo3nix]

I agree with Andy: it's derailleur.

[JudgeJ]

Yes, "derailleur" is the generic name for this type of system. There used to be three main manufacturers, Campagnolo ("Campag") which were the best and most expensive; Simplex, and Benelux. I expect these have either gone out of business or been outgunned by far eastern manufacturers.