The Rear Derailleur is the component which shifts the chain on the rear sprocket. Again, there are many different variations and brands of derailleur, with some designed for mountain bike use and some designed for road bike use. Their design however, at a basic level - remains the same, as does their functionality. Deraillerus arew fixed to the bike with a bolt, which secures the derailleur to the derailleur hanger.
Most mountain bikes and road bikes have both a front and rear dearailleuer, but SRAM pioneered a system which is increasingly used, using a 1 x chainring. The rear derailleur has to work incredibly hard during a bike ride, so it’s important that you choose the right derailleur for your bike, and keep your bike's transmission and drivetrain correctly maintained.