A common carrier in common law countries (corresponding to a public carrier in some civil law systems, usually called simply a carrier) is a person or company that transports goods or people for any person or company and is responsible for any possible loss of the goods during transport. A common carrier offers its services to the general public under license or authority provided by a regulatory body, which has usually been granted "ministerial authority" by the legislation that created it. The regulatory body may create, interpret, and enforce its regulations upon the common carrier (subject to judicial review) with independence and finality as long as it acts within the bounds of the enabling legislation.

A common carrier (also called a public carrier in British English) is distinguished from a contract carrier, which is a carrier that transports goods for only a certain number of clients and that can refuse to transport goods for anyone else, and from a private carrier. A common carrier holds itself out to provide service to the general public without discrimination (to meet the needs of the regulator's quasi-judicial role of impartiality toward the public's interest) for the "public convenience and necessity." A common carrier must further demonstrate to the regulator that it is "fit, willing, and able" to provide those services for which it is granted authority. Common carriers typically transport persons or goods according to defined and published routes, time schedules, and rate tables upon the approval of regulators. Public airlines, railroads, bus lines, taxicab companies, phone companies, internet service providers, cruise ships, motor carriers (i.e., canal operating companies, trucking companies), and other freight companies generally operate as common carriers. Under US law, an ocean freight forwarder cannot act as a common carrier.

The term common carrier is a common law term and is seldom used in Continental Europe because it has no exact equivalent in civil-law systems. In Continental Europe, the functional equivalent of a common carrier is referred to as a public carrier or simply as a carrier. However, public carrier in Continental Europe is different from public carrier in British English in which it is a synonym for contract carrier.

Although common carriers generally transport people or goods, in the United States the term may also refer to telecommunications service providers and public utilities. In certain U.S. states, amusement parks that operate roller coasters and comparable rides have been found to be common carriers; a famous example is Disneyland.

Regulatory bodies may also grant carriers the authority to operate under contract with their customers instead of under common carrier authority, rates, schedules and rules. These regulated carriers, known as contract carriers, must demonstrate that they are "fit, willing and able" to provide service, according to standards enforced by the regulator. However, contract carriers are specifically not required to demonstrate that they will operate for the "public convenience and necessity." A contract carrier may be authorized to provide service over either fixed routes and schedules, i.e., as regular route carrier or on an ad hoc basis as an irregular route carrier.

It should be mentioned that the carrier refers only to the person (legal or physical) that enters into a contract of carriage with the shipper. The carrier does not necessarily have to own or even be in the possession of a means of transport. Unless otherwise agreed upon in the contract, the carrier may use whatever means of transport approved in its operating authority, as long as it is the most favorable from the cargo interests' point of view. The carriers' duty is to get the goods to the agreed destination within the agreed time or within reasonable time.

The person that is physically transporting the goods on a means of transport is referred to as the "actual carrier". When a carrier subcontracts with another provider, such as an independent contractor or a third-party carrier, the common carrier is said to be providing "substituted service". The same person may hold both common carrier and contract carrier authority. In the case of a rail line in the US, the owner of the property is said to retain a "residual common carrier obligation", unless otherwise transferred (such as in the case of a commuter rail system, where the authority operating passenger trains may acquire the property but not this obligation from the former owner), and must operate the line if service is terminated. ^{[citation needed]}

In contrast, private carriers are not licensed to offer a service to the public. Private carriers generally provide transport on an irregular or ad hoc basis for their owners.