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 power unit (engine & transmission) and cab that is used in combination with a semitrailer. When combined together it is labeled as a tractor/trailer unit (commonly referred to as a 'semi'). The trailer is a separate piece of equipment that attaches by way of a 'fifth-wheel' attachment on the back of the tractor. A truck that carries cargo on the same chassis as the power unit and cab (i.e. a single piece of equipment) is commonly called a straight truck. An example of this type of vehicle would be a refuse truck that collects residential waste or a box truck that is used for residential delivery.