The Constitution of the United States divides the federal government into three branches; the Legislative branch (makes laws); the Executive branch (enforces laws); and the Judicial branch (interprets laws). The Constitution gives these branches checks and balances on each other in order to stop any of the three from becoming too powerful. The founding fathers devised this system because they thought that balancing the power between three branches would be the best way to ensure liberty.
The Legislative branch is the branch that makes laws, spends money, levies taxes, declares war, and approves treaties. It is generally thought that the founding fathers intended this branch to be the most important because it was created in Article one of the Constitution. This branch includes the Senate and the House of Representatives, collectively called the Congress as well as some other offices such as the Library of Congress.
The Executive branch is the branch that enforces laws. It consists of the President, Vice President, and the Cabinet (advisors and heads of departments). This branch is unique because all executive power is invested in one person. The president is the head of state, commander in chief of the military and chief legislator. Contrary to popular belief, the Vice President only has one job created by the Constitution, which is to serve as President of the Senate, though President's usually give them additional jobs and duties so they rarely perform that function. The Cabinet is made up of the heads of executive departments, the Vice President, and others.
The Judicial branch is the branch that interprets the laws. It consists of the Supreme court and other lower federal courts. The Supreme Court is the highest court in the land and it is responsible for resolving disputes between states and interpreting the meaning of laws, especially the Constitution. A decision by the Supreme Court is final and can only be overturned by another Supreme Court decision. Additionally, any Supreme Court desition servers as a precedent which is binding to lower courts. Other federal courts, such as trial and appellate courts are created by congress.