All Mountain Bike Parts Descriptions

#14) Cranks - The main drive, from legs to wheels.
  Cranks change your leg power to wheel motion and with the rest of the drivetrain, can be up to seven times more energy efficient than walking. So if I had a great hike and walked 10 km's, I could have rode 70 km's with the same amount of energy.
  They come with single, double, or triple chain rings. Large rings for driving the bike at high torque speeds (low gear) or slower, low torque speeds (high gear). Ring sizes vary from 22 teeth to 50 teeth or even larger for track bikes. Like the bottom brackets they are designed to fit only certain axles shapes and sizes.
  Cranks take alot of abuse, not only from the riders weight on them, but also from smacking off rocks and things, even impacts on the pedals, so they are usually overbuilt with beefy hardened alloys or tubular chromoly steel.

Bike Parts Diagram