By law they are separate and distinct. Trucks are subject to less environmental regulations (emissions, mpg) are allowed to not conform as closely to automobile standards (the reason why you see trucks with the hood above the height of small children, and you need a stepladder to climb in) and also have to pass different crash tests to be considered "road safe" (a truck only has to not annihilate another truck in a crash test, but crash tests aren't done with say a truck and a motorcycle, or a truck and a small car)