The judicial branch of the U.S. government is responsible for interpreting laws as well as determining the constitutionality of laws passed by Congress. It may also make determinations about the constitutionality of executive orders of the president.
Although all federal courts traditionally fall under the umbrella of the judicial branch, the only court specifically prescribed by the Constitution is the Supreme Court. The Supreme Court is composed of a panel of judges selected by the president and then confirmed by the Senate. Congress determines the exact number, since the Constitution does not specify the number of judges that are to sit on the Supreme Court at any given time. The number has varied throughout the years, but the number of justices usually is nine.
Once a Supreme Court judge is confirmed, she cannot be removed from office unless she dies, resigns or is impeached by Congress for wrongdoing. The Supreme Court's duty to interpret laws means that the judicial branch primarily hears only cases that have been initially heard and decided by lower courts but then appealed on the basis of constitutionality. Once the Supreme Court interprets a law, it is expected that lower courts should follow suit in applying precedents set by the Court to similar cases.