The answer lies in whether equipment attaches via a 'fifth wheel' or resides on the same chassis as the cab.
A truck tractor is a combination of a power unit (engine and transmission) and a cab, which is used together with a semitrailer. When they are combined, they are referred to as a tractor/trailer unit, commonly known as a 'semi'. The trailer is a separate piece of equipment that attaches to the back of the tractor using a 'fifth-wheel' attachment.
On the other hand, a straight truck is a type of truck that carries cargo on the same chassis as the power unit and cab, making it a single piece of equipment. For example, refuse trucks that collect residential waste or box trucks used for residential delivery are considered straight trucks.