The Confrontation Clause is a portion of the Sixth Amendment to the U.S. Constitution. It specifies that a defendant in a criminal case has the right to "be confronted with the witnesses against him." It ensures that the defense has a chance to rebut testimony, including that pertaining to any statements made in or outside of court, presented by prosecution witnesses at trial.
A subsequent amendment to the U.S. Constitution ensures that a defendant's rights as stipulated in the Confrontation Clause applies in both state and federal cases.
Consequently, a defendant's rights under the Confrontation Clause are violated if a state or federal court judge severely limits cross-examination. Violations may also occur if a presiding judge allows too much leeway when it comes to the use of statements made outside of court. Specifically, the latter violation occurs if the judge allows such statements to be introduced if the person making them isn't in court to testify.