Oligarchy is a system of government in which power is shared by a privileged few. Theoretically or ideally, the individuals sharing power would be considered the wisest of the populace. The concept of oligarchy descends from the Greeks. The government of Sparta was an oligarchy, while neighboring Athens had a democracy. In contrast to the democracy, Sparta was ruled by a few and not everyone.
There have not been many examples of oligarchies in history or literature. The governing structure of the community in the book The Giver is an oligarchy. Everything is ruled by committee, and the committee is selected based on natural intelligence and wisdom. Power is equally shared, and although there is a Chief Elder that person’s role is mainly as spokesperson for the committee.
The classic definition of an oligarchy is that it is the rule of the few. This is as opposed to a dictatorship (rule of one) or a democracy (rule of the many) or even an aristocracy (rule by a class that gets its power by inheritance). In modern times, the country that is most often referred to as an oligarchy is Russia. You can see examples of Russian oligarchs in the businessweek.com link below.
Media accounts tend to refer to Russia as an oligarchy because of the extreme power of some of the richest men in the country. These are men who have gotten control of huge companies such as those that extract Russia's mineral wealth. These men have come to have a lot of influence with the Russian government because of how rich they are and how much the economy depends on their companies.