The derailleur is an essential part of a bicycle and responsible for changing gears. It moves the chain between the different sprockets so that the bicycle can be cycled efficiently. There are two types of derailleurs: rear and front derailleurs. The rear derailleur is the most advanced and specially designed for multi-speed bicycles. It precisely moves the chain between the sprockets, ensuring smooth and efficient movement. The front derailleur is a simpler mechanism, but still important to the bike. It moves the chain to the correct sprockets and ensures smooth and efficient movement.