differences

A shipment is generally referred to as a collection of goods which at some point will be, are currently, or have already been moved from one geographical location to another.

For Example:

[Will Be]: The shipment will be sent out on Friday. [Are Currently]: The shipment is in transit to its final destination. [Have Already Been]: The shipment arrived yesterday.

Cargo has a possessive implication as in the cargo of a ship or a truck's cargo. A shipment is non-possessive and can generally span multiple shipment methods.

In many cases these terms may be used interchangeably. Shipments is most likely an older term which originally referred to products retrieved from a large cargo ship.