A constitution sets the principles for how a country, state or other political organization is governed. It defines the branches and powers of government and states the rights of citizens. It is often called the supreme law of a nation because it cannot be changed by ordinary laws. It organizes and constrains power, while at the same time allowing for change.
The framers of our Constitution established a Federal democratic republic (a government where the people govern themselves). They created a system to ensure that the Federal Government serves the public, and not private interests. They separated and balanced the powers of the Government among three branches—the executive (invested in the President), the legislative (invested in Congress, the House of Representatives and Senate), and the judicial (invested in the Supreme Court and other federal courts). The Founders also designed the system to be resilient against tyranny by any one branch of government or by any group or individuals within any branch.
Article II
The Constitution delegates many important decisions to Congress, including raising revenue, declaring war, and enacting laws. It also lays out the rules for electing the President (through the Electoral College), and determining his or her eligibility, as well as the terms of office. It also imposes conditions that the judicial and executive branches must abide by when making certain decisions. It requires the advice and consent of the Senate for most key executive and judicial appointments, and for ratifying treaties.