True public goods are those goods which can't be provided to one group of consumers, without being provided to any other consumers who desire them. Thus they are "non-excludable." Illustrations include radio and television broadcasts, the services of a lighthouse, national security, and a clean environment. Private markets specially underinvest in the provision of public goods, as it's very difficult to collect revenue from their consumers. More broadly, public goods can refer to any services or goods provided by government as a result of an inability of the private sector to supply those products in acceptable quantity, quality, or accessibility.