The vast majority of bicycles on the market today use derailleur gears on the front, back or both in order to provide shifting power. They were first used in the 1800s when bicycle enthusiasts in France created the it when inspired by train derailments. Derailleur systems have multiple gears that rotate and a chain that derails, or jumps, from one gear to the next when the bike rider shifts.
For modern bicycles, the derailleur apparatus moves when the bicycle rider shifts the pegs up on the handlebars, the movement transfers to cables running down to the gear system and, ultimately, the chain shifts to the next gear. This derailleur system works to shift either up or down to a larger or smaller gear.
A friction derailleur gear set, which was popular on older bicycles and those for casual, everyday riding, makes the chain move to the next gear and it self-adjusts as much as possible. Performance, racing or mountain bikes sometimes have indexed shifting with derailleur gears. This means that the rider adjusts the chain him or herself after the shift is complete. This can often give a slight performance boost to the ride.
Back it have two gears in a housing or cage that helps move the chain. Front derailleurs have a lever and pulley system in the cage instead. There are various types of it to choose from and more research should be done before replacing or upgrading the gear mechanism on your bike.