When the writers of the Constitution developed our plan of government, they created three branches of government. They gave each branch a different job. This was done so no branch would become too powerful. It also would require the branches to work together, at least to some degree.
The legislative branch is responsible for making laws. Only the legislative branch can do this. This branch is the Congress and includes the House of Representatives and the Senate. When the Affordable Care Act was passed requiring all Americans to have health insurance, only Congress could pass this law.
The executive branch is responsible for carrying out the laws. The President and Vice President are the leaders of this branch. The President is responsible for appointing federal judges and ambassadors. Only the executive branch can do this.
The judicial branch is responsible for interpreting our laws. This branch includes the court system. The judicial branch can determine if laws are legal or unconstitutional. If a law violates the Constitution, the judicial branch will rule that the law is unconstitutional. Only the judicial branch can do this.
Each branch of government has a specific job. Those jobs differ from each other. This keeps one branch from becoming too powerful.