Rear road derailleurs (rear mechs) facilitate moving the chain across the rear cassette or freewheel, the force of pulling or releasing the gear cable from the gear shifter will move the rear mech across the rear sprockets one gear at a time. Rear mechs are available with either short or medium length cages. If you are using a triple chainset you will need a medium cage rear mech to account for the greater chain length, they can also be specific to the manufacturer and amount of rear sprockets so are normally available in 7 & 8 speed, 9 & 10 speed  and 11 speed versions