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. This helps in optimizing pedaling efficiency and speed. 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 cassette. Proper adjustment and maintenance of the derailleur are essential for smooth gear transitions and overall bike performance.