Plutocracy is a type of state government controlled by a wealthy oligarchy. There have been many plutocracies down through history, and the United States is one of them. (More on that below.) This is unsurprising, since wealth can easily be translated into power and power can easily be consolidated for purposes of political control. Also, wealth can easily be used to create more wealth, thus leading naturally to the well-worn dichotomy of the Haves versus the Have-Nots and to the consolidation of political power in the hands of the Haves.
There weren't always state governments, because there weren't always states. We can see forerunners of plutocratic states, though, in native societies which are run by so-called Big Men, tribal leaders who obtained and maintained their position by means of flagrant and competitive demonstrations of wealth.
With the advent of nation states after the agricultural revolution (since somebody had to control the source of irrigation water and the distribution of food between growing seasons), political control was always dependent on personal wealth.
Carthage is a good example of an ancient plutocracy. It was the center of Phoencian civilization after Tyre, the former center, fell to the Babylonians. Early kings of Carthage were military leaders, and the crown was generally available to the highest bidder. For hundreds of years, the "elected" kings largely came from just two families -- two rich families, of course. Fast forward. After Hannibal's 15-year occupation of much of Italy, the Third Punic War was won by the Romans and Carthage was completely destroyed.
The Italian city-states of the Middle Ages were plutocracies, and in many ways they ran the show in medieval Europe. The fall of the Byzantine Empire to the Ottomans changed Venice, Genoa and Florence from merchant empires back into plain old cities. Their dependence on foreign lands and resources was ultimately more expensive and risky than even their richest elite could maintain.
The United States was founded as a plutocracy. In order to vote, you had to own land, thus proving that your wealth was above a certain threshold, that you belonged to the land-owning class. You also had to be a white male in order to vote and you may well have owned African slaves, thus reinforcing the idea that "democracy" was not really a core concern at the time. Gradually, black men were allowed to vote, later even women, of all people. Most recently, young adults who were old enough to have their conscripted asses shot off in ideologically mandated foreign wars were found to be old enough to vote as well. Sadly, the expansion of the franchise has not transformed the country into a democracy.
Against all this historical background, it should come as no surprise that the United States has remained a plutocracy -- a democracy in name only -- since its founding.
If almost the entire Congress, including two opposing candidates for President, votes for a $700 billion dollar bailout of rich, greedy, incompetent bankers even in the face of hundreds of economists' complaints that the bailout would be premature (at best), unnecessary, or disastrously ill-conceived (at worst), then there should be little doubt in anybody's mind that American plutocracy has closed ranks with the support of both major parties.