Ruling class

Come on baby, eat the rich, bite down on the son of a bitch... The term "ruling class" describes the sort of social class that dominates the political, social, and/or economic life of the polity it rules. Wherever there has been civilization there has always been one or more ruling classes. Examples of ruling classes include the Kshatriyas of India, to the centralized aristocracies of pre-WW2 Europe, to the political elites that dominate the United States of America, China and Russia today. Ideally, within a democracy, the entire populace is the ruling class; in reality, all democracies are still dominated by wealth and social privilege.

Note that the existence of a ruling class does not mean that it has total control of a polity (the monarchs of old were deposed, after all) and that therefore there may be multiple, competing ruling classes (such as the old nobles, the nascent bourgeois, and the religious clergy of Renaissance Europe).

Origins
The modern idea of a ruling class, as distinct from a group of individual rulers, stems largely from Karl Marx's writings. Marx spoke of the ruling class, the capitalist bourgeois, being overthrown by the underclass, the proletariat ("Workers of the world, unite!"). A large number of people spent decades making enormous messes trying to implement Marx's vision. Meanwhile, the workers of the world were actually uniting in labor unions and successfully demanding better working conditions and failing, slowly having their plications removed such as the NHS in the UK slowly being defunded.

Abuse of the term
Originally, in Marx's writings, "bourgeois" meant "of, relating to, or characteristic of the owners of the means of production". For leaders of the USSR, "bourgeois" eventually meant the equivalent of "of, relating to, or characteristic of Satan". "Bourgeois art" (art that wasn't conducive to the USSR's goals), "bourgeois pseudoscience" (science that didn't feel pro-communist), and so on were repressed by totalitarian rule. Furthermore, being accused of having a "petty-bourgeois mentality" could mean excommunication from communist circles and sometimes earned one a trip to the gulag or the gallows.