A constitution creates a national government, generally consisting of a legislative, an executive, and a judicial branch, with a system of checks and balances among the three branches. Second, it divides power between the federal government and the states/provinces or regions. And third, it protects various individual liberties of the citizens of the nation.
The American Constitution remains special in this history, but its exact provisions, its elaborate system of checks and balances, its constrained list of rights, and its brevity are not very often copied exactly these days. But the American constitutional experience showed how one might construct a long-lasting democratic government though the design of political institutions. And this model of political creation has captured constitution-writers ever since.