What is the difference between All-wheel drive and four-wheel drive?

All wheel drive - means the power as explained in power goes from engine to all wheels of a vehicle basically the term uses for multi axle heavy duty trucks or trailer tractors.
Four Wheel Drive- as it says power goes to 4 wheel drive only.
4×4 (also, four-wheel drive and 4WD)
Reflecting two axles with both wheels on each capable of being powered.

6×6 (also, six-wheel drive and 6WD)
Reflecting three axles with both wheels on each capable of being powered.
8×8 (also, eight-wheel drive and 8WD)
Reflecting four axles with both wheels on each capable of being powered.