The US Department of Transportation (DOT) regulates, and licenses all freight transportation entities.  The following are the DOT's official definitions.  All freight service providers must be licensed in one of the following categories:        

In government language what most people call a "trucker" or a "trucking company" is called a "motor carrier" or sometimes just a "carrier." The word "company" is used below as shorthand to mean any person, partnership, corporation, etc., engaged in these activities. "FHWA" stands for the Federal Highway Administration, 400 7th St. SW, Washington, D.C. 20590. 

Motor Carrier: A company that provides truck transportation. There are two types of motor carriers, private carriers and for-hire carriers. To operate as an interstate motor carrier, either as a private or as a for-hire carrier, a company must register with FHWA.. 

Private Carrier: A company that provides truck transportation of its own cargo, usually as a part of a business that produces, uses, sells and/or buys the cargo being hauled. 

For-Hire Carrier: A company that provides truck transportation of cargo belonging to others and is paid for doing so. To operate as an interstate for-hire carrier, a company must also register with FHWA.. There are two types of for-hire carriers, common carriers and contract carriers. A for-hire carrier may be both a common and a contract carrier, but must file separate registrations to obtain both licenses. 

Common Carrier Before January 1, 1996: this was a company that provided for-hire truck transportation to the general public. The services offered and the prices charged were published in a public tariff and these were the only prices the common carrier could charge. 

Contract Carrier Before January 1, 1996: this was a company that provided for-hire truck transportation to specific, individual shippers based upon private contracts between the carrier and each shipper, stipulating the services offered and the prices charged to each. 

Current definition "Common" or "Contract": The FHWA requires an applicant to designate whether it is registering as a "common carrier" or a "contract carrier." The historical difference between these two types is reflected in the definitions immediately above. The ICC Termination Act of 1995 defines contract carriage as truck transportation provided under a contract, but, effective January 1, 1996, it no longer distinguishes between common or contract carriers. However, the Act specifically authorizes FHWA to continue registering applicants as either common or contract carriers. The current principal distinction between the two types is that common carrier applicants must file proof of cargo insurance while contract carrier applicants are not required to do so. 

Freight Forwarder: A company that arranges for the truck transportation of cargo belonging to others, utilizing for-hire carriers to provide the actual truck transportation. The Forwarder does assume responsibility for the cargo from origin to destination and usually does take possession of the cargo at some point during the transportation. Forwarders typically assemble and consolidate less-than-truckload (LTL) shipments into truckload shipments at origin and disassemble and deliver LTL shipments at destination. Forwarders must also register with FHWA.. 

Broker: Also a company that arranges for the truck transportation of cargo belonging to others, utilizing for-hire carriers to provide the actual truck transportation. However, the Broker does not assume responsibility for the cargo and usually does not take possession of the cargo. Brokers must also register with FHWA.. 

These trucking definitions are simplified for easier understanding.  Legally acceptable definitions of these activities would be longer, more complex, and supported by the results of many administrative proceedings, court decisions and judicial opinions. If there is any question about the applicability of these simplified definitions, a legal authority should be consulted.