This site about USA politics.

The United States is a federal constitutional republic, in which the President of the United States (the head of state and head of government), Congress, and judiciary share powers reserved to the national government, and the federal government shares sovereignty with the state governments. Federal and state elections generally take place within a two-party system, although this is not enshrined in law.

The executive branch is headed by the President and is independent of the legislature. Legislative power is vested in the two chambers of Congress, the Senate and the House of Representatives. The judicial branch (or judiciary), composed of the Supreme Court and lower federal courts, exercises judicial power (or judiciary). The judiciary’s function is to interpret the United States Constitution and federal laws and regulations. This includes resolving disputes between the executive and legislative branches. The federal government’s layout of is explained in the Constitution. Two parties, the Democratic Party and the Republican Party, have dominated American politics since the American Civil War, although other parties have also existed.

There are major differences between the political system of the United States and that of most other developed democracies. These include increased power of the upper house of the legislature, a wider scope of power held by the Supreme Court, the separation of powers between the legislature and the executive, and the dominance of only two main parties. The United States is one of the world’s developed democracies where third parties have the least political influence.

The federal entity created by the Constitution is the dominant feature of the American governmental system. However, some people are also subject to a state government, and all are subject to various units of local government. The latter include counties, municipalities, and special districts.

This multiplicity of jurisdictions reflects the country’s history. The federal government was created by the states, which as colonies were established separately and governed themselves independently of the others. Units of local government were created by the colonies to efficiently carry out various state functions. As the country expanded, it admitted new states modeled on the existing ones.

From: en.wikipedia.org