The most common layout for modern road cars is to have an engine fixed in the front of the car powering just the front wheels. In a four-wheel drive car (sometimes called all-wheel drive), power is delivered to both the front and the rear wheels, hence ‘four-wheel drive’.
The major use for four-wheel drive is to provide grip – especially off road. Vehicles like the original Jeep and Land Rover were ideal in rugged terrain. If one wheel became stuck in snow, mud or sand, rather than just the other wheel moving things along, all three could help out.
Some sports cars come with four-wheel-drive too, where all four wheels providing grip and traction, helping power the car off the line from standing starts, and being able to drive faster around corners.