Each state is different but here's a basic guideline:
Plumbers work with materials and fixtures used in the installation, removal, maintenance, extension and alteration of plumbing systems. A plumbing system includes the water supply and distribution pipes; plumbing fixtures and traps; soil, waste, and vent pipes; and building sanitary and storm drains including the connections with any point of disposal (public or private) within the property line.
Only a master or journeyman plumber examined and licensed by the Board of State Examiners of Plumbers and Gas Fitters, with the proper permits issued by the local plumbing inspector, can perform plumbing work at your home or business.
Any person licensed as a master or journeyman plumber may perform the work of a gas fitter, but a licensed master or journeyman gas fitter may not carry out the work of a plumber.
A master plumber has worked for a minimum of one year as a journeyman, completed 100 hours of advanced plumbing theory, and successfully passed the master plumber examination. A master plumber may work either as an individual, a firm, or a corporation. A master plumber must maintain a regular place of business where the current license is displayed. The master plumber may employ journeymen and apprentices.
A journeyman plumber has worked for three years as an apprentice, completed 300 hours of theory at an institution approved by the Department of Education and the Board of State Examiners and successfully passed the journeyman examination. A journeyman works as an individual in performing plumbing work. State regulations require that a journeyman carry a license and display it upon request.
An apprentice plumber is a person learning the trade who is registered with the Board and works under the direct supervision of a master or journeyman plumber.