Perjury is making verifiable false statements under oath in a court of law. In short, perjury is lying under oath. Its a crime because the witness is sworn to tell the truth, and that means he/she must tell the truth. This is in order to maintain the credibility of testimony before the court which means it must be able to be relied upon by the finder of fact, which is usually a jury. This is actually a through back to ancient days when swearing upon the Bible or other religious books which were considered sacred. If someone swore to tell the truth and then lied, they would effectively face a far worse fate than simply telling a lie. Telling a lie meant they would have to face God or some other Diety or be denied entrance to Heaven, etc. While much of that no longer plays a major role in today's trials and their witnesses, perjury is still considered a very serious crime because it can lead to the wrong outcome in a trial which is really a search for the truth. It also seeks to take away the authority of the courts by supplanting it with the power to simply lie one's way out of things for their own benefit. Perjury can also apply to those who choose to affirm rather than swear to tell the truth. An Affirmation is used by a person who does not have a religions belief that is bound or determined by the Bible or other Holy scripture, such as an atheist or agnostic. In those situations, they will simply Affirm the truthfulness of their statement instead of swear to be honest.