To understand any country's political system, it is helpful to know something of the history of the nation and the background to the creation of the (latest) constitution. But this is a fundamental neccesity in the case of the American political system. This is because the Constitution of the United States is so different from those of other nations and because that Constitution is, in all material respects, the same document as it was over two centuries ago.
Unlike Britain but like most nation states, the American political system is clearly defined by basic documents. The Declaration of Independence of 1776 and the Constitution of 1789 form the foundations of the United States federal government. The Declaration of Independence establishes the United States as an independent political entity, while the Constitution creates the basic structure of the federal government. Both documents are on display in the National Archives and Records Administration Building in Washington, D.C. which I have visited several times. Further information on the thinking expressed in the Constitution can be found in the Federalist Papers which are a series of 85 articles and essays published in 1787-1788 promoting the ratification of the Constitution.
An important feature of the American political system is that the two major parties - the Democrats and the Republicans - hold a system of primaries to determine who will be their candidate in the general election. These primaries are particularly important when it comes to the four-yearly Presidential election.
To an extent quite extraordinary in democratic countries, the American political system is dominated by two political parties: the Democratic Party and the Republican Party (often known as the 'Grand Old Party' or GOP). These are very old and very stable parties - the Democrats go back to 1824 and the Republicans were founded in 1854.