A Plutocracy comprises a government system where wealth is the principal basis of power (from the Greek ploutos meaning wealth).
The influence of wealth on governance can be expressed either via the wealthy classes directly governing, or (more typically) by the wealthy classes using money to control the government. This control can be exerted positively (by financial "contributions" or in some cases, bribes) or negatively by refusing to financially support the government (refusing to pay taxes, threatening to move profitable industries elsewhere, etc).
There have not been many examples of a "true" plutocracy in history as such, although they typically emerge as one of the first governing systems within a territory after a period of anarchy. Plutocracy is closely related to Aristocracy as a form of government, as generally wealth and nobility have been closely associated throughout history.
In the present era, there are numerous cases of wealthy individuals exerting financial pressure on governments to pass favourable legislation. Most western partisan democracies permit the raising of funds by the partisan organisations, and it is well-known that political parties frequently accept significant donations from various individuals (either directly or through corporate institutions). Ostensibly this should have no effect on the legislative decisions of elected representatives, however it would be a bit idealistic to believe that no politicians are influenced by these "contributions". The more cynical might describe these donations as "bribes", although legally they are not.
See also:
wikipedia.org dumped 2003-03-17 with terodump