Bicycle derailleurs are used to change gear. They work by shifting the chain. On some bikes there is a derailleur on the front chain-rings and one on the rear-sprockets, but most bikes just have a gear-shifter on the rear wheel.
Whether the derailleur is on the front or the back it works by deflecting the chain onto an adjacent chain-ring or sprocket. Rear derailleurs are easier to shift because they are installed on the return path of the chain, where there is the minimum of tension. The front derailleur, on the other hand is most usually installed on top where the chain carries all of the rider's effort.
Adjustment of derailleurs is comparatively simple, though it is most often done by a bike shop. This is mainly because owners are intimidated by the adjustments, and partly because the adjustments need to be done on a storage rack or in a repair stand. For those who can overcome these barriers or improvise with an extra pair of hands, follow the links to the adjustment pages.