A four-wheel-drive car uses a transfer case to split the engine's power evenly between front and rear axles rather than supplying all the energy to one set of drive wheels. This provides better grip and control in certain surface conditions, since all four wheels are powered. These systems are often paired with traction control systems in order to maintain power, even when one wheel spins freely.
Typically, four-wheel-drive systems come in two varieties. Traditional four-wheel-drive cars power two wheels most of the time, as most other cars on the road. The driver has the option of engaging both axles when conditions become tricky, connecting the nondrive wheels to the power train. These systems are not meant to run in four-wheel-drive mode constantly, since they may not have the necessary differentials to handle high-speed cornering with both axles engaged.
All-wheel drive is a permanent four-wheel-drive system, usually paired with several differentials and a traction control system in order to maintain power to the wheels as necessary. Without traction control, a single wheel that is able to spin freely locks the others in place, since the engine's power can escape through the frictionless wheel. Traction control systems apply the brakes if one wheel spins too fast, preventing a loss of power in slick conditions.