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