A derailleur is a mechanical device used in bicycles to change gears. It works by moving the chain between different sprockets on the bike's cassette or freewheel, allowing the rider to select a gear that suits their riding conditions. There are typically two types of derailleurs: the front derailleur, which shifts the chain between the chainrings, and the rear derailleur, which shifts the chain across the cassette. The derailleur is controlled by the rider using shifters located on the handlebars. When the rider shifts gears, cables connected to the derailleur pull or release, causing it to move. This mechanism is essential for optimizing pedaling efficiency and adapting to various terrains, making it a crucial component of modern bicycles.