Among them is also the fire of purgatory, in which the souls of just men are cleansed by a temporary punishment, in order to be admitted into their eternal country, into which nothing defiled entereth. The truth of this doctrine, founded, as holy Councils declare,' on Scripture, and confirmed by Apostolic tradition, demands exposition from the pastor, all the more diligent and frequent, because we live in times when men endure not sound doctrine.
Lastly, the third kind of abode is that into which the souls of the just before the coming of Christ the Lord, were received, and where, without experiencing any sort of pain, but supported by the blessed hope of redemption, they enjoyed peaceful repose. To liberate these holy souls, who, in the bosom of Abraham were expecting the Saviour, Christ the Lord descended into hell.
SOURCE:
http://www.catholicapologetics.info/thechurch/catechism/ApostlesCreed05.shtmlDecree Concerning Purgatory
Since the Catholic Church, instructed by the Holy Ghost, has, following the sacred writings and the ancient tradition of the Fathers, taught in sacred councils and very recently in this ecuмenical council that there is a purgatory,[1] and that the souls there detained are aided by the suffrages of the faithful and chiefly by the acceptable sacrifice of the altar, the holy council commands the bishops that they strive diligently to the end that the sound doctrine of purgatory, transmitted by the Fathers and sacred councils,[2] be believed and maintained by the faithful of Christ, and be everywhere taught and preached. The more difficult and subtle questions, however, and those that do not make for edification and from which there is for the most part no increase in piety, are to be excluded from popular instructions to uneducated people.[3] Likewise, things that are uncertain or that have the appearance of falsehood they shall not permit to be made known publicly and discussed. But those things that tend to a certain kind of curiosity or superstition, or that savor of filthy lucre, they shall prohibit as scandals and stumbling-blocks to the faithful. The bishops shall see to it that the suffrages of the living, that is, the sacrifice of the mass,[4] prayers, alms and other works of piety which they have been accustomed to perform for the faithful departed, be piously and devoutly discharged in accordance with the laws of the Church, and that whatever is due on their behalf from testamentary bequests or other ways, be discharged by the priests and ministers of the Church and others who are bound to render this service not in a perfunctory manner, but diligently and accurately.
SOURCE:
http://www.ewtn.com/library/COUNCILS/TRENT25.HTM#1693 [ De novissimis] * It has likewise defined, that, if those truly penitent have departed in the love of God, before they have made satisfaction by the worthy fruits of penance for sins of commission and omission, the souls of these are cleansed after death by purgatorial punishments; and so that they may be released from punishments of this kind, the suffrages of the living faithful are of advantage to them, namely, the sacrifices of Masses, prayers, and almsgiving, and other works of piety, which are customarily performed by the faithful for other faithful according to the institutions of the Church. And that the souls of those, who after the reception of baptism have incurred no stain of sin at all, and also those, who after the contraction of the stain of sin whether in their bodies, or when released from the same bodies, as we have said before, are purged, are immediately received into heaven, and see clearly the one and triune God Himself just as He is, yet according to the diversity of merits, one more perfectly than another. Moreover, the souls of those who depart in actual mortal sin or in original sin only, descend immediately into hell but to undergo punishments of different kinds [see n.464].
SOURCE:
http://www.catecheticsonline.com/SourcesofDogma7.phpalso
http://www.newadvent.org/cathen/12575a.htm