What does the judicial branch do?
Generally speaking, the judicial branch interprets the laws. It does not make laws and it does not enforce laws. Instead, it interprets them.
In a system like that of the United States, each branch of government has its own function. The legislative branch passes laws. The executive branch enforces them. The judicial branch, by contrast, interprets the laws. What this means is that the judicial branch is the one that decides what the laws mean. When there is a dispute over the meaning of a law (whether it is a law passed by Congress or the Constitution), the judicial branch is asked to decide whose interpretation of the law is correct. Thus, the judicial branch interprets the law by deciding what it means with respect to any particular case brought before it.
The Constitution of the United States divides the government into three branches, the Executive (the President), the Legislative (Congress), and the Judicial. Of these three, the first two are elected and the third appointed. Unlike the other two branches, the Judicial one is intended to be technocratic, in the sense of containing experts in law, rather than merely popular and politically influenced. The purpose of the Judicial branch is to fulfill the mandate of the Constitution to the effect that every person has a right to a trial by jury in front of a competent judge.
The Judicial Branch is comprised of the Supreme Court, United States district courts, and United States courts of appeals. All of these are Federal courts, and can make decisions concerning interpretation of the law that are binding on lower courts. They can rule on either criminal or civil cases, and normally hear those cases in which there is some question as to the interpretation of the relevant laws.
The Supreme Court is in charge of the judicial branch of government according the the US Constitution. They are in charge of interpreting the laws and the Constitution. Since the US Constitution is so difficult, there are justices that are selected to the Supreme Court who must analyze and interpret the Constitution.
The President has the power and authority to appoint a justice to the Supreme Court, but that justice must be agreed upon by Congress. Congress also has the ability to determine the size of the Supreme Court. At one time there were only six judges, while today there are a total of nine. One just serves as the Chief Justice and the other eight are considered to be associate judges.
According to the US Constitution, Congress also has the ability to establish the lesser courts which fall under the Supreme Court. The laws that the Supreme Court rules on become the laws that the lower courts must follow. All decisions made by the Supreme Court are final, while the decisions in the lesser courts can be challenged and may eventually be challenged often enough, that it would end up going to the Supreme Court. The Supreme Court also rules on whether or not the President is following the Constitution as well as any other parts of the government.
Overall, the purpose of the judicial branch is to balance the other branches and determine whether or not their actions are Constitutional. The Supreme Court justices must have a firm grasp of the Constitution. While the Supreme Court usually does not hold typical trials, it does instead interpret cases to determine whether or not a law is being applied correctly and determine if the law is constitutional.