An indulgence is one way for the faithful to reduce the punishment they have to undergo for their forgiven sins. Its root is in the Latin indulgentia, which means a show of kindness, as well as indulgere, which means to forgive.
The process of sanctification requires not only the forgiveness of sins but also purification from the harmful effects of sin. Different actions can help in our purification, including the use of indulgences.
From the Catechism of the Catholic Church 1471:
The doctrine and practice of indulgences in the Church are closely linked to the effects of the sacrament of Penance. “An indulgence is a remission before God of the temporal punishment due to sins whose guilt has already been forgiven, which the faithful Christian who is duly disposed gains under certain prescribed conditions through the action of the Church which, as the minister of redemption, dispenses and applies with authority the treasury of the satisfactions of Christ and the saints.”
“An indulgence is partial or plenary according as it removes either part or all of the temporal punishment due to sin.” The faithful can gain indulgences for themselves or apply them to the dead.
Christians are called to "be perfect, just as your Heavenly Father is perfect." (Matthew 5:48). The process of sanctification requires not only the forgiveness of sins (confession) but also purification from the harmful affects of sin. This is why the Church firmly teaches the existence of purgatory, as "nothing unclean" can enter Heaven (Revelation 21:27).
The Church exists in communion and thus the path to purification is never solitary. Different actions can help in our purification, such as the sacraments, engaging in spiritual works, and, in this case, the use of indulgences.
In current Catholic discipline there are two types of indulgences available: plenary and partial. A partial indulgence is one that partially frees from the punishment due for sins; a plenary indulgence totally frees from the punishment due for sins.
Any member of the faithful can gain partial or plenary indulgences for themselves or apply them for a deceased member of the Christian faithful.
While Church history has admitted one member of the Christian faithful applying an indulgence to another living member of the Christian faithful, this is not currently contained in the Church's canonical discipline (see c. 994).
To gain an indulgence, there are four basic criteria:
Further, plenary indulgences have three additional criteria:
Yes, baptized Catholics can receive partial indulgences if they fulfil the required criteria. However, since plenary indulgences have the requirements of making use of the sacraments, under normal circumstances baptized non-Catholics are unable to fulfil the necessary conditions to obtain the indulgence.
Historically, the Christian faithful had to satisfy this condition as early as noon prior to performing the act and up to eight days following. However, current Church discipline allows either twenty days before or after the specified act.
There are many, many other ways to obtain an indulgence.Some of these are "general grants", meaning they do not include specific actions. These are four-fold and are all partial indulgences:
Plenary indulgences can only be obtained once daily. Further, the act of going to Confession and Communion can only be applied to a single indulgence. This means that if you want to receive a plenary indulgence two days in a row, you must also make two separate acts of approaching the sacraments. The exception to this is in cases of danger of death when the faithful can be given a plenary indulgence by a priest who imparts the apostolic blessing, regardless of whether they already received an indulgence that day.Partial indulgences can be granted as frequently as the faithful make use of them.
The current catalogue of indulgences was last updated in 1999 by the Apostolic Penitentiary and is called the Enchiridion indulgentiarum. This Latin text is available in English translation from the United States Conference of Catholic Bishops.
Blessed Pope Paul VI revised the current discipline on indulgences substantially after the Second Vatican Council in a 1967 document called Indulgentiarum doctrina. This is available on the Vatican website.