A derailleur is a mechanical device used on bicycles to change gears. It works by moving the chain between different sprockets on the cassette and the chainring. This allows the rider to select a gear that best suits their riding conditions, making pedaling easier or harder as needed. There are two main types of derailleurs: the front derailleur, which shifts the chain between the chainrings, and the rear derailleur, which moves the chain across the cogs on the rear wheel. Proper adjustment and maintenance of derailleurs are essential for smooth gear changes and optimal bike performance.