A semi-trailer tractor is a commercial vehicle equipped with special devices for towing semi-trailers.

The front of the vehicle with driving capability is called a tractor, while the rear of the vehicle without driving capability is called a trailer. The trailer is towed by the tractor.

There are two ways to connect a tractor and a trailer: the first is that the front half of the trailer is placed on the traction saddle above the rear section of the tractor, and the bridge behind the tractor bears a part of the weight of the trailer, which is called a semi-trailer; The second type is that the front end of the trailer is connected to the rear end of the tractor, which only provides forward pulling force and drags the trailer, but does not bear the downward weight of the trailer, which is called full trailer.