When a party ships goods by sea, there are two main contracts that will be involved: (1) the underlying sale contract; and (2) the contract of carriage to transport the goods from A to B.  Either the buyer or the seller will organise the contract of carriage, but the carrier might end up in a dispute with either of those parties, so both buyer and seller want to be able to have privity of contract with the carrier. The following diagram shows how that happens. 

The diagram also shows what happens at the financial level. A buyer thousands of miles away from a seller doesn't want to run the risk of non-delivery of the goods (or documents representing title to the goods), so they frequently use banks as intermediaries who give cross-undertaking to each other, which makes the process more secure (although not always!). 

Because this diagram has lots of moving parts, we recommend you download the dynamic PowerPoint presentation below as well as look at this static diagram. It talks you through each stage.