The back wheel is where most of the shifting and braking (for deceleration) will happen.
Terms from diagram above:
- Seat Stays: The seat stays are part of the bicycle frame. They connect the rear wheel to the seat tube.
- Chain Stays: These are also part of the frame and they connect the rear wheel to the bottom bracket/crank.
- Cassette: The cassette, on most quality bicycles, is actually a set of thin "cogs" (basically tiny chain rings). They are stacked horizontally from small to large and vary in number from as few as 5 to as many as 10 separate cogs. The smallest one is the fastest and is for level to down hill riding. The largest is the most powerful and is for climbing up hills.
- Rear Derailleur: The rear derailleur literally de-rails the chain from one cassette cog to the next. It has a hangy-leg-thingy that has two small cog-shaped wheels mounted in its frame. This hangy thing keeps the chain tight so that all the extra slack is taken up to keep the chain from dragging the ground. Having the shifters set in two large gears it uses a lot of chain length to go around gears that large. I digress, the rear derailleur changes gears when you tell it to.
Quick links to specific bicycle sections & parts: