The Supreme Court of the United States constitutes one of the three branches of the U.S. government. The Court stands as the final arbiter of the law and guardian of constitutional liberties. It is powerful, some say the most powerful of the branches, dynamic in its evolution, and political, as has been witnessed many time through its history. Currently the total number of justices sitting on the Court is nine. These positions are appointed.
The Supreme Court consists of the Chief Justice of the United States and such a number of Associate Justices as may be fixed by Congress. The number of Associate Justices is currently fixed at eight (28 U.S.C. §1). As explained on the Supreme Court's website, "Power to nominate the Justices is vested in the President of the United States, and appointments are made with the advice and consent of the Senate. Article III, §1, of the Constitution further provides that '[t]he Judges, both of the supreme and inferior Courts, shall hold their Offices during good Behavior, and shall, at stated Times, receive for their Services, a Compensation, which shall not be diminished during their Continuance in Office.'"
For more about the structure of the Federal Courts, follow the links below (original and complete texts from the Avalon Project at Yale Law School).