Freight carriers are the freight companies that physically transport the freight on behalf of shippers. In most cases they are not also freight forwarders, who manage the shipment on behalf of the shipper; however, NVOCCs are kind of a combination of carrier and forwarder.
If you are making an international freight shipment, the shipment will likely be carried by separate freight companies for pickup, main transit, and delivery. Freight trucking companies and railroad companies carry most pickup, delivery, and other domestic freight. Cargo and standard airlines for air freight, and ocean carriers for sea freight, carry most international freight.