Derailleur gears are a type of gear system commonly found on bicycles. They work by using a mechanism that moves the chain between different-sized sprockets on the rear wheel and sometimes on the front. This allows cyclists to change gears easily, making it simpler to ride on various terrains, whether climbing hills or speeding on flat roads. The system consists of two main parts: the front derailleur and the rear derailleur. The front derailleur shifts the chain between the front chainrings, while the rear derailleur moves the chain across the rear cassette. Together, they provide a wide range of gear options for efficient cycling.