Although Freight and Cargo are most times considered to be the same, especially when referring to goods being imported or exported, there are however some differences between the two. Freight is the term mostly used to describe the volume of goods being transported on a semi trailer of a train or truck.
This simply explains why there are vehicles like freight truck and freight train. Also, the term freight can also be used for the money you pay for transportation when goods are transported to you. On the other hand, Cargo is the term mostly used when the transportation of goods is done via ship or plane.
It is rare to see people using the term freight for goods that are being transported via ship or plane. This also explains why there are vehicles like cargo planes and cargo ships. Unlike the term freight that can be used for the money paid on the transported goods, this doesn't go with cargo.