Article III of the Constitution establishes the Judicial Branch, but gives Congress the discretion to shape its composition and adapt rules to meet evolving needs. Congress also has the ability to create courts, which include district courts and appeals courts. Federal courts bear the responsibility of interpreting the law, determining whether a specific law conforms to the Constitution, and applying the law to specific cases, typically ones that are disputed. The Judicial Branch only intervenes in actual cases and controversies, rather than having the power to interpret laws in general if they are unattached to a specific case.
The highest court in the nation is the Supreme Court, and the justices remain in office until they die, resign or are impeached or convicted by Congress. Decisions made by a standing Supreme Court cannot be appealed to any higher authority. It has the final say on matters of federal law in the United States.