What a loaded question...where to start?
The US government is made up of three branches. It was thought by our founding fathers that this would help to provide a system of checks and balances, so that one branch does not hold all of the power.
The Legislative Branch is Congress. We have a bicameral legislative branch, meaning that we have two "houses." (The reason we have 2 is because they were trying to figure out how to base representation, population or an equal number from each state, they compromised with one of each.) This branch's primary responsibility is to make laws.
Next we have the Executive Branch, aka the President. The President is responsible for executing the laws. He must sign the laws passed by Congress (even though Congress may vote to override it if he doesn't sign...). He also has the power over the agencies that make sure that the laws are being carried out.
Finally, we have the Judicial Branch (aka the Supreme Court). The SC is made up of nine "justices." Their primary responsibility is to hear cases about relevent federal issues that are certioraried to them (basically appealed...but they are allowed to deny cases, they don't have to hear them...). Another important thing that the Supreme Court has the power to do (thanks to an early court case) is interpret and review the Constitution. The Constitution is our Supreme Law, and the Court has the power to look at it and determine "what it means, and what it means not."
The three branches are, in theory, equal.
That's the basic structure of the US government...I don't know if that answered your question or not...
"Never doubt that a small group of committed people can change the world. Indeed it is the only thing that ever has." --Margaret Mead