A public utility is a company that maintains the infrastructure for a public service (often also providing a service using that infrastructure). Public utilities often involve natural monopolies, and as a result are often government monopolies, or if privately owned, treated as specially regulated sectors.
Public utilities can be privately owned or publicly owned. Publicly owned utilities include cooperative and municipal utilities. Municipal utilities may actually include territories outside of city limits or may not even serve the entire city. Cooperative utilities are owned by the customers they serve. They are usually found in rural areas. Private utilities, also called investor owned utilities, are owned by investors. Unlike public companies, private utilities may be listed on the stock exchange. Private, in this context, means not owned by the public or the government.