New document by the American Bishops.
I found the following, from the Appendix interesting:
i. What are indulgences?
An indulgence does not confer grace. An indulgence is not a remission of the guilt due to sin. The guilt due to sin is ordinarily taken away by the Sacraments of Baptism and Penance (confession), in which we receive forgiveness for sins through Jesus Christ. Although guilt is taken away, and with it the eternal penalty that is due to sin—namely, damnation, the eternal loss of the presence of God—there remain consequences for sins that those who have committed them must bear. There is what is traditionally called the temporal punishment for sin.
By its very nature, every sin inevitably causes suffering for the one who has committed it. Every sinful act creates a disorder within the soul of the human person; it distorts our desires and affections, leaving us with "an unhealthy attachment to creatures, which must be purified either here on earth, or after death in the state called Purgatory."57 Furthermore, sin disrupts one's relationships with God, with the Church, with other people, and with the world as a whole. The communion intended by God is damaged or lost. Those who have received forgiveness for their sins still have an obligation to undergo a difficult and painful process (the temporal penalty for sin) to be purified of the consequences of their sins and to restore the disrupted relationships. "While patiently bearing sufferings and trials of all kinds and, when the day comes, serenely facing death, the Christian must strive to accept this temporal punishment of sin as a grace."58 The necessary and painful process that brings restoration and purification can take place either in this life or in Purgatory, as whatever part of the process remains unfinished at death must be completed in Purgatory.59
Through an indulgence, God grants the prayer of the Church that the temporal penalty for sin due to someone be reduced (or possibly eliminated). By God's grace, participation in a prayer or action that has an indulgence attached to it brings about the necessary restoration and reparation without the suffering that would normally accompany it. The granting of an indulgence by the Church is "the expression of the Church''s full confidence of being heard by the Father when—in view of Christ's merits and, by his gift, those of Our Lady and the saints—she asks him to mitigate or cancel the painful aspect of punishment by fostering its medicinal aspect through other channels of grace."60