A Truck is the truck that does not trail a trailer or a semitrailer and the tractor is the same truck but pulling a trailer or a semitrailer. The part in the back is called a semitrailer because only the back end sits on wheels (and thus "trails"); the front end is supported by the tractor (The truck). A full trailer is fully supported by its wheels at front and back. A truck need not have a trailer to be considered a truck.