Judicial Review is the power of the Courts to declare Acts of Congress or the States as well as of the President or other governmental officials to be contrary to the meaning and intent of the Constitution. There is no specific provision in the Constitution to provide for Judicial Review, but the power was inferred by Chief Justice John Marshall in the case of Marbury vs. Madison.
The powers of the legislature are defined, and limited; and that those limits may not be mistaken, or forgotten, the constitution is written. To what purpose are powers limited, and to what purpose is that limitation committed to writing, if these limits may, at any time, be passed by those intended to be restrained? The distinction, between a government with limited and unlimited powers, is abolished, if those limits do not confine the persons on whom they are imposed, and if acts prohibited and acts allowed, are of equal obligation. It is a proposition too plain to be contested, that the constitution controls any legislative act repugnant to it; or, that the legislature may alter the constitution by an ordinary act.
Between these alternatives there is no middle ground. The constitution is either a superior, paramount law, unchangeable by ordinary means, or it is on a level with ordinary legislative acts, and like other acts, is alterable when the legislature shall please to alter it.
If the former part of the alternative be true, then a legislative act contrary to the constitution is not law: if the latter part be true, then written constitutions are absurd attempts, on the part of the people, to limit a power, in its own nature illimitable
[T]he particular phraseology of the constitution of the United States confirms and strengthens the principle, supposed to be essential to all written constitutions, that a law repugnant to the constitution is void; and that courts, as well as other departments, are bound by that instrument
Since the judges in this nation are sworn to uphold the Constitution and are bound by its terms, any act or law found to be violative of the Constitution is unenforceable by the Courts. The Courts thus have a powerful check on the other branches of the government as they have the ability to stop their actions completely. Importantly, if the courts determine that laws or acts violate the Constitution, the Courts decision can only be reversed by amending the Constitution. There have been many attempts to amend the Constitution to circumvent Court holdings, but they tend to fail uniformly.