Derailleur Gears:

Derailleur gears work by having several cogs at the front and back of different sizes, and use a cage mechanism (the derailleur) to “derail” the chain from one cog to another. It sounds unsophisticated and it is, but it is light and works surprisingly well. Derailleur systems are exposed to the elements so need a reasonable amount of maintenance. They can have 5, 6, 7, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 24, 27 or even 30 gear combinations, although there is quite a bit of overlap in the ratios.