What is a Rear Derailleur?

Definition: Your rear derailleur is the mechanism that causes your bike to change gears in the rear. It is a spring-tensioned device that moves the chain between the different sprockets on your cassette. Two pulley wheels and a spring-driven arm work to move the chain between gears when you move the shifter in the front. The rear derailleur and the front derailleur work independently of one another.