The term shifter might not be completely accurate, according to the late cycling expert Sheldon Brown. The part of the bicycle that does the actual shifting is the internal hub systems or bicycle derailleurs – the shifter simply controls the action. It is a smooth process : a bit of clicking, twisting and tapping and your bike shifts in no time. There are different kind of shifters on different bikes
Trigger shifters These are usually a standard on mountain bikes and hybrids. Unlike road shifters, which work with drop handlebars, trigger shifters come with upright handlebars and they can work in more than one way. Usually, you either pull, tap o push a small lever in order to shift a gear. Sometimes there is only one lever to control all of the shifting, and sometimes these functions are separate.
Internal hubs This is technically not a shifter, but we do need to mention it. The internal gear systems are usually controlled by either grip shifters or simple triggers. With internal gear systems, the bikes gears are located inside the hub, which is not the case with bikes equipped with a traditional derailleur. The hub is where all of the spokes are connected. It is a very good idea to enclose all of the gears because this protects them from the elements and possible debris on the road. The big difference is that, with an internal hub, you have the luxury of changing gears and controlling the shifters while at a standstill, or while coasting – this is not the case with other systems, where you need to pedal in order to shift gears.
Road Bike Shift Levers Road shifters come on classical Race style road bicycles. This mechanism is usually integrated with the brakes on a racing bike and the shift lever doubles as the brake lever as well. In order to change gears, you need to move the brake lever centrally, towards the bike (the right shifter controls the rear, and the left one controls the front gears). There is sometimes and additional lever under the brake lever and its function is to shift the bike in the other direction of the shift lever, which is the same as the brake lever. The top company that manufactures bicycle components is Shimano and their best shifter model is the Dura-Ace. There are also Tiagra and Sora, which are not as high end but also worth mentioning.
These are usually a part of comfort oriented bikes, cruisers and hybrids. Like their name says, they are a part of the grips on your bike. Just in case- the grips are the rubber things you put onto your handlebars in order to provide better traction and comfort. You can see grip shifters at the ends of your grips, centrally. The gear shifting is simple, you just move the grip according to your needs. Pushing it once equals shifting the gear once in any direction. Their main advantage is the fact that they are simple to use because you hardly have to change your position in order to switch gears on your bike.