The shipper will guarantee that the cargo is put on board a shipping service which the recipient hires. Since "FOB" is typically only quoted in reference to very heavy and/or bulky items, the implication is that the originator will load the shipment, but arranging for the actual transportation is up to the recipient.
FOB shipping terms are typically quoted as a location; for example, "FOB Anchorage, Alaska" -- meaning that the shipment will be loaded at that location. This is to help the recipient determine the cost and most efficient method of shipping to be used.
Compare c.i.f (cost, insurance and freight).