In the first theory, the Marxist Theory, it is believed that the large corporations control or influence what happens in the government. This ideology is based on the writings of Carl Marx, but there are many different versions or Marxism, all arguing on the specifics of power separation.
The next theory is the Power Elite Theory. In the book The power Elite, C. Wright Mills describes that there is a three divisional group that controls the government. The three parts are the corporate leaders, the top military officers, and a handful of political leaders.
The third theory is the Bureaucratic theory. Max Weber who is credited for the theory believed that Marx had ignored the social and political fact of modern times. He had the idea that the large government agencies and other bureaucracies were the true starters and holders of political power.
The last and final theory is the Pluralist point of view. There is no specific person that has been given credit for the ideology, but it believes that power mainly relies on major interest groups. Over all though, the idea is that no one area has complete control and it is impossible for a certain part of the economy or government to rule any whole country.
In todays world, I believe that the Pluralist Theory is the most prevalent in todays society. In order for a political party of group to gain power in our government, they need support buy many other companies and interest groups. Government used to be staffed by party hacks. Today, it is staffed by people from public policy schools (New York times). That comment means that even in todays society, even a school teacher can have a little bit to do with running the government.