Two forms of constitutional review: abstract review tests a law's constitutionality in general, often at the request of political actors and without a dispute; concrete review arises from an actual case before a court.
In more detail
Abstract review is typical of specialised constitutional courts on the Austrian or Kelsenian model, such as Germany's Federal Constitutional Court, where a federal or state government or a quarter of the Bundestag can request it. Concrete review is the norm in the American model following Marbury v. Madison, where any court may refuse to apply an unconstitutional law in a case. Abstract review can prevent harm before a law is applied, but critics say it draws courts directly into partisan conflict.