A tractor is simply a large truck (semitrailer) that is pulled between the front of the truck and the front of the truck by tools (that is, the front of the truck can be pulled away from the front of the truck and other cars can be pulled away from the front of the truck and other cars)
The front has the driving capacity of the front is called the tractor, the rear does not have the traction drive capacity of the car is called the trailer, the trailer is towed away by the tractor. There are two ways to connect the tractor and trailer: the first is that the front half of the trailer sits on the traction saddle above the rear segment of the tractor, and the bridge behind the tractor bears part of the weight of the trailer, which is the semitrailer; The second is that the front end of the trailer attached to the back end of the tractor, tractor only provide forward pull, towing the trailer to go, but not bear the weight of the trailer down, which is fully attached.