A Future for Socialism
In economic jargon, a public good is one from which people can't be excluded; if the good is available to anyone, it's available to all. Classic examples are clean air and water, and not having your country invaded. Getting a higher wage because of collective bargain also qualifies, at least for all the employees of a given firm. It's very hard for markets or other distributed, voluntary mechanisms to provide adequate supplies of a a public good, because of the inevitable temptation to take a free ride. (This is discussed more formally and with great insight in Mancur Olson, Jr., The Logic of Collective Action.) A public bad, by analogy, is something unpleasant which everyone is forced to suffer, the classic example being pollution.