The three branches of government are the executive branch, legislative branch, and the judicial branch.
The executive branch is headed by the president. He wears many hats. He carries out federal laws, directs national defense, and negotiates foreign policy. He also has the power to veto laws that are proposed, and he can also propose laws.
The legislative branch is run by congress. The Congress is made up of the House of Representatives and the Senate. Congress has 100 senators - two from each state. The Vice President is considered the head of the senate. There are 435 representatives. The number of congressmen is based on the population of the state. The purpose of both groups is to create laws. Its powers, as you would expect, include passing laws and impeaching officials.
The judicial branch is headed by the Supreme Court. Its jobs include interpreting the Constitution, reviewing laws, and deciding cases involving rights.
As you can see there is a division of power in the government. The benefit of this is that there are checks and balances.