A more thorough explanation:

An able seaman is a skilled and experienced sailor who is qualified for all seaman's duties and certified by an inspecting authority. They contribute to the function of a navigating vessel or the accomplishment of its mission. They are attached to a navigating vessel as an employee below the rank of officer.

Under the Jones Act and the Longshore and Harbor Workers' Compensation Act, a person who is an able seaman and is injured (or killed) ‘in the course of his employment’ is covered. Seamen's injuries are also covered under the general maritime law.

For example, if a ship is transporting goods over water, the able seaman will be responsible for manning, reefing, and steering the vessel. If the ship is operating as a cruise ship, the able seaman will be responsible for other duties such as assisting passengers and maintaining the ship's facilities.

Another example is in the exploration for oil and gas on navigable waters. The amphibious oil workers aboard floating drilling structures are considered able seamen and are entitled to the seaman's remedies against their employers and the operators of the ‘vessels' on which they are employed.

In contrast, an ordinary seaman has some experience but is not proficient enough to be classified as an able-bodied seaman. A merchant seaman is a sailor employed by a private vessel, as distinguished from one employed in public or military service.