By his death and Resurrection, Jesus opened heaven (CCC 1026). Prior to that time all who died went to "hell"; however, the just went to a place in hell referred to as "the Bosom of Abraham," where they would be comforted. The parable of the Rich Man and Lazarus (Luke 16:19-31) seems to indicate that there were two parts of hell. Both Lazarus and the rich man died and went to hell, but Lazarus was comforted in the bosom of Abraham while the rich man was in a place of torment. A great chasm separated the two parts.
The Catechism explains,
Scripture calls the abode of the dead, to which the dead Christ went down, "hell"— Sheol in Hebrew or Hades in Greek—because those who are there are deprived of the vision of God. Such is the case for all the dead, whether evil or righteous, while they await the redeemer: which does not mean that their lot is identical, as Jesus shows through the parable of the poor man Lazarus who was received into "Abraham’s bosom": "It is precisely these holy souls, who awaited their Savior in Abraham’s bosom, whom Christ the Lord delivered when he descended into hell." Jesus did not descend into hell to deliver the damned, nor to destroy the hell of damnation, but to free the just who had gone before him. (CCC 633)
For a fuller explanation, see "Hell? Yes! Part I"