The president is the chief executive and is charged with enforcing the laws. The heads of all the government departments report to him. He is responsible for managing the government and spending the money that congress appropriates. He is the commander in chief of the armed forces but is a civilian. He can, with the advice and consent of the Senate, negotiate treaties and appoint justices of the Supreme Court and judges of lower courts and government officials.
The constitution requires that there be a Supreme Court and leaves it to congress to create lower courts. It specifies what kind of cases the Supreme Court has original jurisdiction over and which ones have to be appealed to it from lower courts.
The legislature is the congress. It has two houses, the Senate and the House of Representatives. Each state gets two senators but the number of Representatives is determined by population. All laws are made by the congress. In order to become a law a bill has to be approved by both houses and then signed by the president. If he does not sign it then it doesn't become law unless two thirds of both houses vote to override him. The senate must approve all treaties, all judges nominated by the president, and many federal officers. Once appointed, judges serve for life and federal officers serve at the pleasure of the president. The congress can pass laws only exercising those powers given it by the constitution. All other powers are reserved to the states.
A really important part of the system is what are called checks and balances. Each branch of the government has certain limited powers which are checked and balanced by the other branches. This prevents any one branch from becoming all powerful. There are also checks and balances between the federal government and the states. So long as the power of the government is limited then it can not take away people's fundamental rights. What our founders tried, and succeeded in doing was creating a government which was powerful enough to deal with the problems of a country as large as this one but sufficiently limited that it could not threaten liberty.
In a nutshell that is how the Constitution works. Our founding fathers were geniuses, they came up with something that may seem simple to some but really is a complex document that regulates our everyday lives. It is of the utmost importance in regulating this country and keeping it together. It gives I and the people of the United States the rights and our freedoms that we use every single day of our lives.