Definition:
The front derailler is the mechanism that causes the chain to shift between different sized chainrings on the bicycle's crank up at your feet and pedals.
The front derailleur is usually held in place by tension from a spring; the rider controls its position (and what gear she rides in) by moving shift levers on the handlebars. This causes the derailleur to move, which in turn prompts the chain to move from one chain ring to the next.
