The Church teaches that “each man receives his eternal retribution in his immortal soul at the very moment of his death, in a particular judgment”. St. John of the Cross (1542-1591) wrote, “At the evening of life, we shall be judged on our love”. Perfect love will make possible entrance into heaven, imperfect love will require purification, and a total lack of love will mean eternal separation from God.
“The Church gives the name Purgatory to [the] final purification of the elect, which is entirely different from the punishment of the damned”. Those who die in the state of friendship with God but who are not fully purified and perfected are assured of their eternal salvation. However, they must undergo a purification to obtain the perfection of love and holiness needed to enter heaven, where they have a heart that is totally open to him. This process is called Purgatory.
It is impossible for us to imagine what Purgatory is. Traditionally, it has been described as a purifying fire. Since the human soul cannot be touched by earthly flames, the image serves to recall that perfect love is achieved by a gradual and painful spiritual detachment from selfishness and self-centeredness. The Church assists those in Purgatory through prayer and especially the Eucharist in their final process of purification. Offering Masses for the deceased is the most powerful way of aiding them. November 2 of each year, the Commemoration All the Faithful Departed (All Souls Day), is a day for special remembrance and prayer for the dead.”
 Catechism of the Catholic Church, no 1022
 Dichos, no. 64
 CCC, no 1031
Excerpts taken from pages 153-154 in United States Catholic Catechism for Adults. Copyright © 2006, United States Conference of Catholic Bishops,Washington,D.C. Used with permission.