A constitution sets the principles by which a state is governed. It defines the main institutions of a state (including the executive, legislature and judiciary), places limits on their exercise of power, and guarantees rights and obligations for its citizens.
The document embodies the ideals of its creators and contains detailed instructions for how to run the government. It also establishes a system of checks and balances that ensures that no one branch of the government gets too much power. Most important, it guarantees the individual rights of citizens of the United States.
Article I vests the legislative powers of the United States in a Congress, which is to consist of a House of Representatives and a Senate. The bicameral Congress was designed as a compromise between the larger states, who wanted representation based on population, and the smaller ones, who wanted equal representation for all.
Article II details the Executive Branch of the federal government, laying out rules for electing the President through the Electoral College and for the qualifications and term lengths of the Vice President and Cabinet. Article III grants the judicial powers of the federal government to a Supreme Court and other courts created by Congress. The Framers understood the potential for corruption in the exercise of this power, and so they included clauses that prohibit ex post facto laws (punishing conduct that was legal at the time of its performance) and bills of attainder (singling out individuals or groups for punishment). The Supreme Court interprets these clauses.