In a plutocracy, the people are ruled by the wealthy few. A plutocracy is very different from a democracy, in which in person's vote counts equally.
Whenever you see the suffix -cracy, you know you're dealing with a form of rulership or government. The first part of the word comes from the Greek ploutos, meaning wealth. Put them together, and you get plutocracy, a government ruled by the rich. How does this differ from, say, an aristocracy? Well, the truth is that it isn't very different. Members of the aristocracy tend to be rich, but their money tends to be "old money." In a pure plutocracy, even the overnight billionaire can be a ruler.