CO-BROKERING; 2 brokers working together to move a customers freight. One broker has the load, the other broker claims to have the truck. Both make a commission (usually) and often neither the actual customer nor the customers broker know who hauls the load. You have 3 parties involved in 1 load, not counting the customer.
DOUBLE BROKERING; A broker (or a carrier) sells a load to a carrier thinking that carrier will haul the freight. This carrier, for whatever reason, legitimate or nefarious, sells that load to another carrier. Again, neither the actual customer, nor the customer's broker knows who is hauling the freight. Any communication between the first broker, or his customer, and the actual carrier has to go through the double broker. Any payment problems between the actual carrier and the double broker can have a negative impact on the first, unsuspecting broker and his customer.
INTERLINING; a common practice across North America in the transportation industry. A very transparent and above board procedure of moving a shipment between 2 or more licenced carriers on the same Bill of Lading from origin to destination. Parties to the transaction are known to each other and controlled, usually through contractual arrangements.
In addition to whatiship stated, sometimes a bad carrier will do the same as a bad broker and accept any and all freight offered to him, knowing from the start that the freight will have to be sold off. I guess that is the difference between good carriers/brokers and bad ones. The good ones only accept freight they can handle themselves and service it properly. The bad ones take everything and just hope for the best as far as outcomes are concerned.