Welcome to 4WDS : 4 by 4, Four Wheel Drive Systems.
Genenrally, when a manufacturer says that a vehicle is a 4x4, 4 Wheel Drive System, or 4WD, they generally refer to a part time 4WD system which can be operated as either a 4 or 2 wheel drive system. All wheel drive is only required for slipery or high traction surfaces such as ice or loose metal. There are also permanent 4WDS, but this system is employed in special utility vehicles only.
A 4x4 system gives power to all four wheels so that each wheel gets the maximum traction capacity, here is a simple diagram showing 4WDS.