What is an Indulgence?

According to the Catechism of the Catholic Church:

"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."(Catechism 1471)

There are two types of indulgences:

"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.(Catechism 1471)

A plenary indulgence can only be acquired once a day except to the faithful in danger of death who cannot be assisted by a priest to bring them the sacraments and impart the apostolic blessing with its attendant plenary indulgence (according to canon 468, 2 of the Code of Canon Law).  A partial indulgence can be acquired more than once a day, unless there is explicit indication to the contrary. (Enchiridion of Indulgences 1968)

Indulgences can be applied to oneself or those who are no longer living.  No one acquiring indulgences can apply them to another living person.