Cycle derailleurs, both front and rear, are critical components of a bicycle's drivetrain system, responsible for shifting the chain between different gears on the chainrings (front derailleur) and cassette or freewheel (rear derailleur).
The front derailleur is responsible for moving the chain between the different chainrings attached to the crankset. When the rider shifts gears using the shifter, the front derailleur moves the chain from one chainring to another, allowing for variations in gearing ratios.
The rear derailleur shifts the chain between different sprockets on the cassette or freewheel attached to the rear wheel. It works in conjunction with the shifter to move the chain across the various gears to provide different pedaling resistances.