Index and Friction
These days, the majority of the shifting on mountain bikes is via index shifters. These shifters ‘click’ on every move and the amount of pulling on the cable is very precise.
Friction shifters are simple pullers and they don’t click. They rely on the rider knowing how much to turn the shifter.
These days you will find 3 types of shifters:
These are the most common derailleur shifters today. They have two triggers, one for going up in gears and one for going down.
They are the most important piece in the shifting process as they must provide the precise pulling amount for the derailleur to shift.
They are part of the grips. They shift the gear by turning the shifter back and forth, similar to how motorcycles work.
Shimano recently released a new shifter known as XTR Di2. The shifting is done by a small motor that stays behind the mech in the derraileur and the power is passed through a thin cable.
The battery is shared between the shifter, derailleur and display and the charge lasts for at least a few months as reported by Shimano.