A four-wheel drive, also called 4×4 or 4WD, is a two-axled vehicle drivetrain capable of providing torque to all of its wheels simultaneously. It may be full-time or on-demand, and is typically linked via a transfer case providing an additional output drive shaft and, in many instances, additional gear ranges.
The Jeep Wrangler (pictured is a TJ Wrangler) is a 4WD vehicle with a transfer case to select low-range or high-range four-wheel drive.
Center transfer case sending power from the transmission to the rear axle (right) and front axle (left)
The Lamborghini Murciélago is an AWD that powers the front via a viscous coupling unit if the rear slips
The HMMWV is a 4WD/AWD that powers all wheels evenly (continuously) via a manually lockable center differential, with Torsen differentials for both front and rear
A drivetrain or Transmission System, is the group of components that deliver mechanical power from the prime mover to the driven components. In automotive engineering, the drivetrain is the components of a motor vehicle that deliver power to the drive wheels. This excludes the engine or motor that generates the power. In marine applications, the drive shaft will drive a propeller, thruster, or waterjet rather than a drive axle, while the actual engine might be similar to an automotive engine. Other machinery, equipment and vehicles may also use a drivetrain to deliver power from the engine(s) to the driven components.
Rear axle with hypoid bevel gear final drive
Front wheel drive manual transaxle, showing the gearbox and final drive incorporated in the same housing
Construction vehicle drivetrain, with permanent all-wheel drive
A cutaway view of an automotive final drive unit, which contains the differential