STI (Shimano Total Integration) is the Shimano name for their proprietary shifting components which combine the shifting mechanism with the handlebar mounted brake levers, a combination also commonly known as brifters.
The brakes on STI components are operated in the same manner as traditional brake levers, where pulling the brake lever back towards the handlebar activates the brake. Shifting is accomplished by either pushing the main brake lever in a lateral motion (towards the center line of the bike) in order to pull the derailleur cable, or by pushing a smaller lever mounted behind the main brake lever in order to release cable.
Shimano STI Shifter repair
Individual replacement parts for STI components do not appear to be available for purchase directly from Shimano.