Four-wheel-drive and all-wheel-drive seem to be the same thing, but there are differences in application and in the varying types of vehicles that use them. While all four wheels receive power in order to increase traction, that is where the similarity ends.
Four-wheel-drive systems are found mainly in pickup trucks and larger, off-road SUVs. They function by utilizing a split differential and a lower gear ration. Used mostly for off-road and rougher terrain driving, these vehicles do very well in such conditions.