Gear levers are cable operated levers which move the front or rear mech which in turn moves the chain to a different cog, ie change gear. They pull the gear cable a certain distance, or on modern electronic systems this is done by battery, which moves the mech a corresponding set distance to another cog. This indexing of the gears means no more searching for gears as every time you hear a click the gear is changed.
Gear levers have come a long way from the first rod and lever actuated design in the early 1900's. They moved to their traditional spot on the down tube of the bike shortly after and stayed there until the mid 1990's. Nowadays on road bikes the gear levers are mounted as a combined system with the brake levers on the handlebars. Either the brake lever itself moves left or right, depending on which side, or a lever just behind it depending on which system you are using. Each movement of the lever moves the mech a set distance so indexing carries on making gear change and selection easier.
Each road groupset manufacturer (Shimano, Campagnolo and SRAM) has a different method of shifting but all are housed with the brake levers on the handlebars.
Another constant is rear gears are always changed using the right hand brake/gear lever and front gears are always changed using the left hand brake/gear lever nomatter the make or system so at least that bit is easy to remember.
Shimano calls their system STI (Shimano Total Integration) and in most cases uses the main brake lever to change gear; on the right hand side pushing left makes it easier but on left hand side pushing right makes it harder, and a secondary lever just behind the brake lever to return it the opposite way. Two differences to this norm -
Sora which uses the brake lever and a small lever on the brake hood instead of a secondary lever behind the brake lever
Di2 (electronic shifting) where you only have to touch one of two small buttons found on the outside of the brake lever on each side which thankfully follows the same shifting patterns as normal mechanical cable systems.
Campagnolo calls their system Ergo Power which uses a lever behind the brake lever to change gear; on the right hand side pushing left makes it easier but on left hand side pushing right makes it harder, and a tiny lever on the brake hood to return it the opposite way. The only slight difference to this is their electronic version which uses tiny pads on the levers to change gear by touch rather than moving entire levers.
SRAM call their system Double Tap which uses a single lever behind the brake lever to control the up and down shift on each side. Pushing the lever inwards once changes gear a certain way, on the right hand side pushing left makes it easier but on left hand side pushing right makes it harder, but pushing further inwards until you hear a second click reverses the gear change, ie a double tap to change the other way. All SRAM gear/brake levers use the is system without exception.
Unfortunately, Shimano, Campagnolo and SRAM gear systems are incompatible with each other because each gear system has a certain amount of movement from gear levers and differs slightly between manufacturer. Each manufacturer also recommends only using similar groupset components each gear system as some mechs may not work with differing numbers of gears, ie Shimano Dura Ace 11 speed levers must use an 11 speed rear mech and so on.
The hierarchy of Shimano gear/brake levers from professional to entry level is as follows:
The hierarchy of Campagnolo gear/brake levers from professional to entry level is as follows:
The hierarchy of SRAM gear/brake levers from professional to entry level is as follows: