Brake levers and shift levers at good prices from starbike.com
Shift levers actuate the front derailleur and the rear derailleur which change the gears.
The gear change is transmitted via a tensioned cable to the derailleurs, which then move inwards or outwards.
Shift levers are available as trigger shifters or twist shifters. The latter ones are popular among many mountainbike riders.
The brake levers operate the bicycle brakes and are always handlebar-mounted. There are mechanical brake cable systems and hydraulic disc hoses.
You can find a wide selection of different brake and shift levers, e.g. from Campagnolo, Shimano or SRAM.