You could start here for background infromation: http://en.wikipedia.org/wiki/US_Government

There are effectively only two parties: "Democrats" and "elephants" (more commonly known as "Republicans"). This polarization comes about because in the US people* watch a lot of television, as a result they are mostly incapable of thinking or making choices.

Traditionally, the Democratic party represented the "working class", aligned themselves with unions, favored increasing taxes and larger government; whereas the Republican party represented individualism, freedom from government, less taxation, and a smaller government with less bureaucracy. These positions have been blurred in recent history; for example, the current Republican party is actually creating a much bigger and more powerful bureaucracy.

If you've read this far, you're better informed than a quarter of the voting American populace. Feel good about yourself

The United States is a:
Constitutional
Federal
Democratic
Republic.

 US Constitution is the "supreme law of the land." Constitutional governments are limited in power. They follow the rule of law and due process. They are differentiated from various forms of tyranny, dictatorship, absolutism, and totalitarianism.

The US Constitution is above conflicting State law and above other federal laws like U.S. Statutory and U.S. Administrative law. It outlines the basic structure and functions of our national government.

Federalism is a division of power between the national and state levels of government. It means the US is neither a unitary system nor a confederation.

Democracy refers to political systems where the rulers are popularly elected by a majority of the voters. The franchise refers to who has the right to vote. Today the franchise has been given to all US citizens above the age of eighteen, who have registered to vote and who who are literate. In George Washington's day, the franchise was limited to property owners who paid the requisite property taxes. Women, slaves, and males who did not own sufficient property could not vote. Were we a democracy in George Washington's time?

Republic refers to a political system that is not a monarchy or other hereditary form of government. In a Republic, the rulers are elected in some fashion. A republic is not necessarily a democracy. But republics carry the seeds of democracy within themselves. If the franchise is enlarged to include most adults, then the system becomes a democracy. Republics, in addition to not being monarchies, usually provide for a mixed constitution of various checks and balances. Simple rule by a majority is usually rejected. The tyranny of the majority is to be feared as much as the tyranny of one person or the tyranny of factions.