What are Indulgences?

From New St. Joseph Baltimore Catechism (Nihil Obstat; Imprimatur; 1962)

435. What is an indulgence? An indulgence is the remission granted by the Church of the temporal punishment due to sins already forgiven.

436. How many kinds of indulgences are there? There are two kinds of indulgences, plenary and partial.

437. What is a plenary indulgence? A plenary indulgence is the remission of all the temporal punishment due to our sins.

438. What is a partial indulgence? A partial indulgence is the remission of part of the temporal punishment due to our sins.

439. How does the Church by means of indulgences remit the temporal punishment due to sin? The Church by means of indulgences remits the temporal punishment due to sin by applying to us from her spiritual treasury part of the infinite satisfaction of Jesus Christ and of the superabundant satisfaction of the Blessed Virgin Mary and of the saints.

440. What is the superabundant satisfaction of the Blessed Virgin Mary and of the saints? The superabundant satisfaction of the Blessed Virgin Mary and of the saints is that which they gained during their lifetime but did not need, and which the Church applies to their fellow members of the communion of saints.

441. What must we do to gain an indulgence for ourselves? To gain an indulgence for ourselves we must be in the state of grace, have at least a general intention of gaining the indulgence, and perform the works required by the Church.

442. Can we gain indulgences for others? We cannot gain indulgences for other living persons, but we can gain them for the souls in purgatory, since the Church makes most indulgences applicable to them.