What many people consider to be the pedals on a bicycle are actually a series of important parts that propel the bike forward. The pedal itself is the platform on which a cyclist's feet rest. The pedal is then attached to a crank arm that connects the pedal to the bottom bracket, which is a series of bearings inside the frame that allow the crank arms to rotate. While this sounds somewhat complicated, the system itself is quite simple and allows for a smooth pedal stroke that propels the rider forward.
The crank arm is usually made of steel, aluminum, or higher end materials such as carbon. Inexpensive bikes will have a simple steel crank arm that is very strong and much heavier than other materials, and more expensive bicycles will use lighter materials to save weight. It is important for this part to be very stiff so that pedaling power is not lost to flexing crank arms; instead, the power should transfer directly to the chain, which propels the bike forward using a series of gears.
A crank arm set contains two arms. The non-drive side crank arm, or the crank arm opposite the chain, is a simple arm that connects the pedal and the bottom bracket. The drive side crank arm, however, is a bit more complex: it holds the pedal at one end just like the non-drive crank arm, but at the other end, the arm must attach to the bottom bracket and it must attach to chainrings, or the front "gears" of a bicycle. These chainrings are one component of the drivetrain, which is the system that propels a bike forward. It includes the chainrings, the chain, and the rear cassette--or cluster of gears.
In recent years, the design of crank arms has changed considerably. The method by which the pedals attach to the crank arms has remained largely the same, but the way that the crank arms connect to the bottom bracket has gone through many design changes meant to make the system more efficient and less susceptible to power loss due to flex. The crank arms have historically attached to the bottom bracket by a square hole that attaches to a square tapered arm on the bottom bracket; the crank arms are then secured by a threaded crank arm bolt. Many varieties of fastening systems exist, from splined bottom bracket systems to through-axle systems that are lighter and stiffer.