A drivetrain is the collection of components in a vehicle that drive the vehicle forward. It works in conjunction with a power supply — or engine — to move the wheels or other means of propulsion. On a car, the drivetrain includes the transmission, the driveshaft, the axles, and the wheels. On a simpler vehicle such as a bicycle, the drivetrain includes the chain, the front chainrings, the rear gear cassette, and the rear wheel. A drivetrain is also known as a power train, and the components in the power train are most often all positioned behind the engine and head back toward the rear of the vehicle. The engine is sometimes considered a part of the drivetrain as well.
The idea behind a drivetrain is to transfer power from the engine to the parts of the vehicle that make contact with the ground. The engine produces the power through combustion, which in turn powers a spinning flywheel on the rear of the engine. The transmission makes contact with this flywheel, and can control the amount of power or torque transferred to the rest of the drivetrain. A driveshaft is a long, straight shaft that runs from the transmission to the rear axle; the driveshaft spins, delivering power to a differential in the rear axle.
A differential basically transfers power from the driveshaft to the wheels. The driveshaft is perpendicular to the rear axle, so the power must be somehow diverted from the perpendicular driveshaft to the horizontal axle; a differential accomplishes this by connecting two rods with gears on the end to a gear on the end of the driveshaft. As the driveshaft turns, so too does the differential, spinning the wheels. This is a simple description of how these components work, as there are other parts of the system that have an integral function in propelling the vehicle forward.
Motorcycles and bicycles use a chain instead of a driveshaft to propel the vehicles forward. A bicycle is unique in that the rider is essentially part of the drivetrain: he or she is the power unit that turns the gears to propel the bike forward. Like a car's transmission, a bicycle features a gear-changing system that consists of components called derailleurs. These derailleurs move the chain from one gear to another, changing the rate of propulsion of the bike. On a car or truck, the stick shift and clutch achieve this function in a different way, but to the same end.