As we said in speaking of the judgment, those who have turned against God and are not in a state of grace at the time of death are condemned to hell. There can be no change of heart concerning God, for or against His will, after death. Hence hell and heaven must both be without end.
The chief suffering of hell is the loss of God. In this life, we can go comfortably without thinking of Him. But then it will be different. For one thing, our senses now keep telling us this world and this life are the only important things. Then that din of the senses will be gone. But more especially, when we cross into the next life, as it were, the light goes on. In this life, our intelligence has two components, the spiritual intellect proper to the spiritual soul, which is tied to the marvelous, yet material instrument in our heads. The latter limits us greatly. But at death, that limit is gone. Then even if the soul does not at once see God, it carries with it the information on Him, but then really understands, and wants Him intensely. To lose Him forever, or to be in a twisted state of wanting Him, yet in revolt against Him--this is the chief pain of hell.
Scripture often speaks of fire in hell. On May 17, 1979, the Sacred Congregation for the Doctrine of the Faith explained: "She [the Church] believes that there will be eternal punishment for the sinner, who will be deprived of the sight of God, and that this punishment will have a repercussion on the whole being of the sinner." There will be, then a bodily pain. The imagery of fire means it will be a suffering as intense as that caused by earthly fire.
Of course, those who have sinned more will suffer more. But for all, there is no end to suffering and despair.
Taken from The Basic Catholic Catechism
PART FOUR: The Apostle's Creed VI-VIII
Seventh Article: "From thence He shall come to judge the living and the dead"
By William G. Most. (c)Copyright 1990 by William G. Most