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