An indulgence is a removal of the punishment from sin. Although you are forgiven in Confession for sins the punishment stills remains, which would have to be conducted through purification like in purgatory. If an indulgence is done that part or all of the punishment is removed.
And remember, indulgences are only possible because of God's love displayed on the Cross. Without Jesus Christ, we would have no chance to be forgiven and obtain salvation.
A universal indulgence is granted anywhere in the world while a local indulgence applies to only a specific place or area. A perpetual indulgence is one that may be gained at any time while a temporary indulgence only is available for certain times, for example, like certain indulgences for the Holy Souls in November. A plenary indulgence is the complete remission of the temporal punishment of sin.
Temporal punishments only “cancel out” a certain amount, of which only God knows. If more temporal punishment remains, more indulgences or time in purgatory (which is also biblical) is required in order to reach the perfection of Heaven. I highly suggest a person read the Primer of Indulgences if he/she has questions.
Please also realize that many older documents like holy cards and prayer books would have a certain length of time printed on them. For example, a prayer could say it is a 300 day partial indulgence. However, realize that time does not exist in purgatory or Heaven or hell. The Church has never taught that if such a prayer was said, the person would get 300 days off of purgatory time. An indulgence is only reduced as God sees fit. In order to clarify this very complicated matter, the Church just stopped putting a length of time on partial indulgences following Vatican II in 1969. The dates corresponded only to early Church practices. Since it was so misunderstood, it was changed to allow people to understand it better.
The Catechism of the Catholic Church states the following:
"An indulgence is obtained through the Church who, by virtue of the power of binding and loosing granted her by Christ Jesus, intervenes in favor of individual Christians and opens for them the treasury of the merits of Christ and the saints to obtain from the Father of mercies the remission of the temporal punishment due for their sins." The Church does this not just to aid Christians, "but also to spur them to works of devotion, penance, and charity" (CCC 1478).
The Catechism of St. Pius X states the following:
124 Q. What is an Indulgence?
A. An Indulgence is the remission of the temporal punishment due on account of our sins which have been already pardoned as far as their guilt is concerned — a remission accorded by the Church outside the sacrament of Penance.
125 Q. From whom has the Church received the power to grant Indulgences?
A. The Church has received the power to grant Indulgences from Jesus Christ.
126 Q. In what way does the Church by means of Indulgences remit this temporal punishment?
A. The Church by means of Indulgences remits this temporal punishment by applying to us the superabundant merits of Jesus Christ, of the Blessed Virgin and of the Saints, which constitute what is known as the Treasure of the Church.
127 Q. Who has the power to grant Indulgences?
A. The Pope alone has the power to grant Indulgences in the whole Church, and the Bishop in his own diocese, according to the faculty given him by the Pope.
128 Q. How many kinds of Indulgences are there?
A. Indulgences are of two kinds: plenary and partial.
129 Q. What is a plenary Indulgence?
A. A plenary Indulgence is that by which the whole temporal punishment due to our sins is remitted. Hence, if one were to die after having gained such an Indulgence, he would go straight to Heaven, being, as he is, perfectly exempt from the pains of Purgatory.
130 Q. What is a partial Indulgence?
A. A partial Indulgence is that by which is remitted only a part of the temporal punishment due to our sins.
131 Q. Why does the Church grant Indulgences?
A. In granting Indulgences the Church intends to aid our incapacity to expiate all the temporal punishment in this world, by enabling us to obtain by means of works of piety and Christian charity that which in the first ages Christians gained by the rigour of Canonical penances.
132 Q. What is meant by an Indulgence of forty or a hundred days or of seven years, and the like?
A. By an Indulgence of forty or a hundred days, or of seven years and the like, is meant the remission of so much of the temporal punishment as would have been paid by penances of forty or a hundred days, or seven years, anciently prescribed in the Church.
133 Q. What value should we set on Indulgences?
A. We should set the greatest value on Indulgences because by them we satisfy the justice of God and obtain possession of Heaven sooner and more easily.
134 Q. Which are the conditions necessary to gain Indulgences?
A. The conditions necessary to gain Indulgences are: (1) The state of grace (at least at the final completion of the work), and freedom from those venial faults, the punishment of which we wish to cancel; (2) The fulfilment of all the works the Church enjoins in order to gain the Indulgence; (3) The intention to gain it.
135 Q. Can Indulgences be applied also to the souls in Purgatory?
A. Yes, Indulgences can be applied also to the souls in Purgatory, when he who grants them says that they may be so applied.
136 Q. What is a Jubilee?
A. A Jubilee, which as a rule is granted every twenty-five years, is a Plenary Indulgence to which are attached many privileges and special concessions, such as that of being able to obtain absolution from certain reserved sins and from censures, and the commutation of certain vows.
The conditions for an indulgence are as follows:
- Person must be in the state of grace by the completion of the indulgence.
- The person must also want to gain the indulgence.
- The above conditions as well as the following:
- One is free from all attachment from sin, even venial sins
- One receives the Sacraments of Penance and the Eucharist (Within 7 days of the indulgence)
- One prays for the intentions of the Pope (Apostles Creed, 1 Our Father, and any other prayers)
Raccolta (Listing of Indulgences in Latin and English):
- Also see a listing of prayers with the ones with indulgences marked
- There are indulgences for reading the Bible
- Some Rosary Indulgences
- The Stations of the Cross have a plenary indulgence attached (see bottom of link's page for more information on obtaining the indulgence)
- New Years has special Indulgences attached
- Holy Thursday has a special indulgence for praying "Tantum Ergo"
- Indulgences for All Saints Day, All Souls Day, and November
- Indulgences for Pentecost
- Indulgence for the Feast of Portiuncula (August 2nd)
- Indulgence for the Solemnity of Christ the King
- "A plenary indulgence is granted to the faithful, who in the solemn liturgical action of Good Friday devoutly assist at the adoration of the Cross and kiss it" (Source)
- "A plenary indulgence is granted to the faithful, for the recitation of En ego, o bone et dulcissime Iesu after reception of Communion on a Friday during Lent before an image of the crucified Christ" (Source)
- "A plenary indulgence is granted to the faithful, who devoutly receive the Papal Blessing to the City and to the World (Urbi et Orbi), even if by radio or television"
- "A plenary indulgence is granted to the faithful, who spend at least three days in spiritual exercises during a retreat"
- "A plenary indulgence is granted to the faithful, who recite The Act of Reparation to the Sacred Heart of Jesus on the Solemnity of the Sacred Heart"
- "A plenary indulgence is granted to the faithful for visiting a church or altar on the day of its dedication"
- Plenary Indulgence at the Hour of Death
(More information on indulgences.)