Derailleurs are the gear changing mechanisms that allows you to change gears on multispeed bikes. Derailleurs are a vital part of a bikes drivetrain assembly which allows the bike to shift and go.
There are two types of derailleurs: Front derailleurs and Rear derailleurs. Front derailleurs change gears at chainset making larger gear changes and rear derailleurs shift through the cassette or freewheel attached to the back wheel making smaller gear changes.
The three main brands who make derailleurs are Shimano, Sram and Campagnolo.
There are some upcoming brands such as Microshift, TRP and Box who also make some exciting new drivetrain components.