Governmental powers are specifically assigned to one of the three branches of government created by the Constitution. The other two branches of government may not then perform governmental functions that are not within their realm of responsibility. For example, the Executive Branch may not vote on the budget for the government because that is assigned to the Legislative Branch. The Legislative Branch is not allowed to determine the constitutionality of laws; the Judicial Branch interprets laws and their adherence to the principles of the Constitution.
Along with dividing the work of the government among the three branches, the Constitution also created methods for each branch to control the powers of the other branches. The Executive Branch may veto bills passed by the Legislative Branch, which prevents the Congress from enacting laws that the Executive Branch doesn't want to become active; the Legislative Branch can attempt to override the Executive veto if it strongly wants the bill to become law. The Executive Branch is responsible for nominating persons to serve in responsible positions as Cabinet members or ambassadors, for example, but the nominees must be approved by the Senate (as part of the Legislative Branch) before they can assume their office and powers. The Judicial Branch is responsible for determining if controversial laws are written in agreement with the guidelines and requirements set out in the Constitution, and may declare a law unconstitutional if they find it does not support the Constitution; the Legislative Branch may rewrite the law, taking into account the Supreme Court's interpretation and arguments, as another bill and attempt to get that bill enacted and approved as a new law.
These and many other procedures allow each branch of the government to check the other branches of the government, creating a balance of power by preventing any one branch from becoming too powerful.
The system of checks and balances can be found throughout the Constitution in the relationships between the different branches of government. For example, the President holds a check over Congress through the veto power. But Congress can overturn the veto if 2/3 of them vote to do so, and they can impeach (in the House of Representatives) and vote to remove the President from office (in the Senate.) Few powers, indeed, are held by any branch independently from any other branches. The Senate, for example, checks the President's diplomatic powers by having approval power over treaties with foreign nations. The federal judiciary has the power to rule on the constitutionality of laws, but the President appoints its members, and the Senate must approve them. Congress passes laws, and the President must sign (or, as noted above, veto) them.