The Congress is divided into two chambers: The House of Representatives and the Senate. Both houses have to approve a bill for it to become law. The President then must sign it. If he does not (veto) the law does not pass, but Congress can override the President with a 2/3 vote on any given law. Laws must also be consistent with the constitution, however. If Congress made a law that said "the rights of free speech are hereby abolished", or "The Official Religion of the United States is now Catholicism", these go explicitly against items in the U.S. constitution. The judicial branch has (some would say has taken) the power to declare a law unconstitutional if it goes against the Constitution. The way it is now, the Supreme Court is the ultimate arbiter in these cases. Changing results from the court would either require a constitutional amendment, or a re-ruling by the Supreme Court, presumably with different members, years later. Nothing in the constitution explicitly gives the courts this law overturning power, but it has generally been upheld and was common practice in law at the time. The degree to which judges can modify laws based on unconstitutionality is subject to much debate at the present time.