One of the seven sacraments, also known as the "sacrament of dying," administered by a priest to a baptized person who begins to be in danger of death because of illness or old age, through prayer and the anointing of the body with the oil of the sick.  The proper effects of the sacrament include a special grace of healing and comfort to the Christian that is suffering the infirmities of serious illness or old age, and the forgiving of the person's sins.

(taken from Catechism of the Catholic Church)