When the founders were creating a new American government, the abuses of power from a single ruler were fresh in their minds. They didn't want another king! So the United States government was designed to prevent any single person or group from becoming too powerful. We call this idea "separation of powers." The Constitution separates the government's power into three branches. Each branch has separate responsibilities, but they all must interact with each other in different ways. This separation of powers includes a system of checks and balances, which are ways that each branch can limit the others.
The Legislative Branch is responsible for writing laws, or legislation. We call the legislative branch Congress.
Congress has two chambers: the Senate and the House of Representatives. To pass legislation, both the Senate and the House must approve a bill with a majority vote. (Though recently, owing to the United States' politically polarized climate, bills sometimes need a super majority.) This two-chamber system is an example of checks and balances within a single branch of government.
When the founders were trying to figure out how to best represent citizens in Congress, there was an argument between the small states and the large states. Large states wanted representation based on population to have a louder voice in Congress. But states with smaller populations balked: Why should the large states get more power? They were in favor of an equal number of representatives per state so that they wouldn't get overshadowed. But then the large states thought that was unfair... What to do? A two-chamber system was the compromise. The Senate has two senators from each state, regardless of the size of the state. The number of representatives in the House, on the other hand, is based on population size. This two-part system ensures that the smaller states still have their voices heard, but that the people and their interests are fairly represented.
Congress is responsible for legislation, but the President has to sign a bill before it can become law. He also has the option to veto a bill. That's when he rejects the bill and sends it back to Congress. A Presidential veto isn't necessarily the end of it, though. If there is enough support for the bill, members of Congress can override the veto with a ⅔ majority vote. And it's all part of the checks and balances plan. The President can check Congress's power with a veto, but Congress can check the President back with an override.
The Executive Branch is responsible for ensuring that laws are carried out, or executed.
The President has the most power in the Executive Branch. The branch also includes the Vice President, the Cabinet, and other federal agencies.
The President is the head of the government, but an additional duty of the President is to be the Commander in Chief of the armed forces. That means that he has the final say in military plans.
The President is responsible for appointing people to the Cabinet. It's made up of the fifteen heads of the executive departments: Agriculture, Commerce, Defense, Education, Energy, Health and Human Services, Homeland Security, Housing and Urban Development, the Interior, Justice, Labor, State, Transportation, Treasury, and Veterans Affairs. Additionally, the President appoints judges to the Supreme Court. Appointments to both Cabinet and judicial positions must be confirmed by the Senate, creating a check on who the President can appoint.
While the Legislative Branch creates laws and the Executive Branch carries them out, the Judicial Branch evaluates the fairness of the laws. In other words, they judge.
The Supreme Court is the highest court in the land, and it has the final say when it comes to federal law. When laws in different states conflict, the Supreme Court settles the argument.
To try and keep the Supreme Court out of the political fray, justices are not elected by the people. Instead, they are nominated by the President and confirmed by the Senate. In addition, justices do not serve limited terms of two, four, or six years, the way the members of the Legislative and Executive Branches do. They get to serve until they retire or die. And if justices do something illegal, the senate has the power to impeach them. It's all about the checks and balances.
The Supreme Court has the power to review laws and rule them unconstitutional. So if an unfair law gains the approval of Congress and the President's signature, the Supreme Court can remove it to protect citizens' constitutional rights.
Impeachment is when a federal official is accused of breaking the law. The Legislative Branch can impeach members of the Executive and Judicial Branches. In cases of impeachment, the House of Representatives accuses the official, who is then tried by the Senate.