In your message to me you said these are the three models of power that you're referring to:
1:The Plural Model/The people Rule
2:The Power-Elite Model/A Few People Rule
3:The Marxist Model/Bias in system itself
I'll answer with reference to these:
The pluralist model states that there are plenty of ways that people can influence government. Everyone who is interested in some issue can have a chance to have their voice heard. The people who will win on a given issue are those who care most.
So this model says that the American system is very open and democratic. The main name connected with this idea is Robert Dahl whose book "Who Governs?" is a classic statement of this idea.
The power-elite model is most connected with C. Wright Mills who argued that there was a power elite of people who dominated political, military, economic and social institutions. These people all came from the same background and shared the same values.
This model is similar to the first only in that it agrees that America is democratic. However, it believes that this democracy is much more closed than Dahl says it is. We truly have a democracy, but its rules tend to give the rich more power.
The third model argues that the whole US system works for the upper class to keep workers down. This theory does not have any one major proponent on the level of Dahl or Mills. This model does not see America as democratic but rather as being run by the upper class through its control of economics.
In this view all the trappings of democracy are just window dressing to make a basically exploitative system look good. So this view agrees with Mills that a power elite rules, but it believes the elite does so by coercion and trickery.
This is a rather broad question, since there are many types of power. Here are three:
1. The most obvious model of power is the government and the military. They have the money as well as the physical power to fight and defend. This is a power that is feared not only in America, but in the whole world.
2. There is also the power of knowledge. People in "the know" create knowledge, categories of thought, and what error is. Michel Foucault has written extensively on this topic with great insight.
3. There is the power of money. Since most people in society value money, the more one has, the more power one also has.