What exactly are 18-wheelers, semis, and tractor-trailers?

“Tractor-trailer,” “18-wheeler,” and “semi” are different names for a semi-trailer truck. This rig consists of a towing engine or tractor with a semi-trailer to carry freight, which is attached just forward of the rear axle of the tractor so that a large portion of the weight is carried by the tractor. Hence, the name “semi-trailer,” as it does not trail completely behind the tractor. These are the big rigs that are employed commercially to haul freight.