All Mountain Bike Parts Descriptions

#18) Front and Rear Derailleurs - The mechanical devices that move the chain side to side.
  The Front Derailleur, located above the crank, pushes the bike chain side to side using two steel plates (usually made of steel for durability). It also acts as a chain guide, helping keep the chain going over rough terrain. Most Front Derailleurs are clamped to the bike at the seat tube, and some are fastened to the bike on a bracket around the Bottom Bracket.
  The Rear Derialleur, located below the cog set on the rear wheel, guides or "derails" the chain side to side under the cogs to get your higher or lower gears. It also acts as a spring, pulling back on the chain as it changes gears which, as well, changes the chains length. The extra chain is soaked-up between two pulley wheels below the main spring. Because the Rear Derailleur sticks out the side of the wheel, it is common for them to be impacted on, wacked by closing doors (in apartments or houses), or rocks, or totally crumpeled up into your rear wheel from something jammed in your chain or a bent dearailleur hanger. These mishaps are not covered under warranty so be careful how you lay your bike down. Most aluminium frames use break away pieces hangers on the Rear Derialleur. They are designed to break first if you derailleur gets snagged, hopefully saving it. They are also soft and tend to bend causing out-of-sinc shifting. If you want to check the alignment of the Rear Derialleur, it's easier to see if you are standing behind the bike. The two pulley wheels should be directly below the cog. If they are not, you get to discover two new friends "skip" and "spoke-wedgy".

Bike Parts Diagram