Indulgences are various acts of piety, offered by the Church, which the Church claims offer remission of temporal punishment due to forgiven sins in virtue of the merit of Christ and His Church. Fundamentals of Catholic Dogma defines an indulgence as "the extra-sacramental remission of the temporal punishments of sin remaining after the forgiveness of the guilt of sin." In other words, the guilt for sins may have been removed by the atonement of Christ and the sacraments, but the punishment for them is not: "We can be forgiven [our sins], yet still have to suffer."

There are two kinds of indulgences. First, those which are "easy" to perform (called partial indulgences) and second, those which are more difficult to perform (called plenary indulgences). The difference concerns how much punishment for sin they remove. Plenary indulgences remove all temporal punishment due to sin while partial indulgences only remove part of the punishment due to sin. "This punishment may come either in this life, in the form of various sufferings, or in the next life, in purgatory. We don't get rid of here [through indulgences] what we suffer there [in purgatory]."

The Catholic Encyclopedia explains the rationale for indulgences when it declares,

The remission of the temporal punishment due for sins and hence, the satisfaction owed to God for one's sins is called an indulgence... indulgences granted by the Church may be gained for oneself or for the souls in purgatory.... The granting of indulgences is founded upon three doctrines of Catholic faith: the treasury of the merits of the communion of saints, Christ himself, and the Blessed Virgin and the saints.

Catholicism maintains that indulgences are necessary because it holds that when people sin, God's justice requires a punishment beyond that given to Christ on the cross. Amends must be made to God by some means, whether by indulgences or through the miseries and trials of life, or through death, or through purifying sufferings in purgatory. Indulgences are thus offered to the faithful as a means to escape God's punishment.
Again, on what basis does the Catholic Church offer such indulgences? On the basis of the merit of others having special spiritual quality: Because Christ, Mary and the relatively small number of Catholic saints have all provided "superabundant satisfactions" to God through their merits, the Church believes it can offer these same merits to the Catholic believer in exchange for remission of punishment.

In other words, because of the infinite merits of Christ plus those of the Virgin Mary and the Catholic saints, the Church argues it has the prerogative to administer the benefits of these indulgences "in consideration of the prayers or the pious works undertaken by the faithful."
Supposedly, the requirements for a plenary indulgence are tougher than for a partial. Thus, plenary indulgences (remitting all temporal punishment for sin) were offered to those who partook in the Crusades against the Muslims.

But on the other hand, today, merely performing the Sign of the Cross is an indulgence having great merit:

The Sign of the Cross is the most frequently used sacramental of the Church. The sign is a repetition in motion of the symbol of our salvation, the cross on which Christ died.... The faithful, as often as they devoutly sign themselves with the sign of the cross, are granted an indulgence of three years; whenever they make the same holy sign with blessed water, they may gain an indulgence of seven years.

Devotion to Mary and the Rosary are also "highly indulgenced" by Roman Catholicism:

While the devotion to the Blessed Mary through recitation of the Rosary is recommended for all, it is strongly advised for all who study for the priesthood One of the many indulgences attached to the devotion [of Mary] is that the faithful who recite the Rosary together in a family group, besides the partial indulgence of ten years, are granted a plenary indulgence twice a month, if they perform this recitation daily for a month, go to confession, receive Holy Communion, and visit some Church or public oratory.

Indulgences are also granted for visits of the faithful to various Catholic shrines, some of which were built in honor of Marian apparitions.

But indulgences are received only by performing the specific work and/or requirements to which the indulgence is attached. For example, three conditions are necessary to gain a plenary indulgence in addition to performing the work itself: sacramental confession, eucharistic communion, and prayer for the Pope-as well as the absence of all willful attachment to sin-even venial ("minor") sin.

Under proper conditions, indulgences are actually applied to the dead in purgatory for the reduction of the degree of suffering and the length of time it must be endured.

The modern Catholic Church has made it clear that if a person bought an indulgence merely to get some soul out of purgatory or for buying forgiveness of sins, that this was wrong. The Church never officially condoned the massive abuses of the practice, although it did clearly share in the responsibility for them.

Nevertheless, indulgences can play an important role in the salvation both of individuals on earth as well as in purgatory after death. Still, it must be remembered that this remission of punishment due sin only occurs after the guilt and eternal punishment of sin have been remitted by adherence to Catholic practice. Thus, "The Church grants such indulgences after the guilt of sin and its eternal punishment have been remitted by sacramental absolution or by perfect contrition." Once again, when we see a teaching like this-that the "guilt of sin and its eternal punishment" are remitted by the sacraments-we can only question Catholicism's commitment to the full efficacy of Christ's atonement.