A canonization is a formal declation that a deceased person is a saint.
The process of canonization has its root in the ancient church, but has gone through many changes. In the Catholic Church it is now a rigid process carried out first by the Congreations for the Causes of Saints, and the approval of the Pope.
A canonization is preceded by a beatification, the main difference between the two beign that the canonization is considered infallible while the beatification says that the person is most likely a saint.