There's not much difference between cargo and shipment. Cargo can be on anything: planes, trucks, ships, trains, etc. Cargo represents the tangible goods being moved. For example one can say, "The cargo is a collection of electronics." Shipment (which can be in any mode of transportation) represents the movement of cargo in a way - that is to say cargo can be in a shipment. For example, "The shipment of electronics will be delivered by plane." In today's world, CARgo has nothing to do with CARS; and SHIPments have nothing to do with SHIPS. Hope this helps! :)