destroy the reputation and honor of one's neighbor. Honor is the social witness given to human dignity, and everyone enjoys a natural right to the honor of his name and reputation and to respect. Thus, detraction and calumny offend against the virtues of justice and charity.
While detraction can cause great damage through telling the truth, calumny is, if anything, even worse, because it involves the telling of a lie (or of something that one believes to be a lie). You can engage in detraction without intending to do damage to the person you are discussing; but calumny is by definition malicious. The point of calumny is, at the very least, to lower the opinion one person has of another person.
Calumny can be even more subtle and insidious. The Catechism of the Catholic Church notes (para. 2477) that a person is guilty of calumny if he, "by remarks contrary to the truth, harms the reputation of others and gives occasion for false judgments concerning them." The person who engages in calumny does not even have to specify an untruth about another; all he has to do is place doubts about that person in the minds of others.
While truth is not a defense against the charge of detraction, it is against the charge of calumny. If what you have revealed to someone about a third party is true, you are not guilty of calumny. If the person you revealed it to has no right to that information, however, you are still guilty of detraction.
Calumny goes hand-in-hand with gossip, yet, while we often think of gossip as a venial sin, the Catechism says (para. 2484) calumny is so serious that it can amount to a mortal sin, if the lie that you tell causes grave damage to the person in question:
The gravity of a lie is measured against the nature of the truth it deforms, the circumstances, the intentions of the one who lies, and the harm suffered by its victims. If a lie in itself only constitutes a venial sin, it becomes mortal when it does grave injury to the virtues of justice and charity.
Once you have told a lie about another person, you are morally obligated to try to repair the damage you have done. As the Catechism notes (para. 2487), this applies even if the person about whom you have told the lie has forgiven you. That reparation may be much more than simply admitting that you have lied. As Father Hardon notes,
[T]he calumniator must try, not only to repair the harm done to another's good name, but also to make up for any foreseen temporal loss that resulted from the calumny, for example, loss of employment or customers.
The magnitude of the reparation must match the magnitude of the offense, and, according to the Catechism of the Catholic Church (para. 2487), the reparation may be "sometimes material" as well as moral. To use Father Hardon's example, if your lie has caused someone to lose his job, you may even be obligated to make sure that he can pay his bills and feed his family.
Like detraction, calumny is rarely ever a minor sin. Yet the most seemingly innocuous gossip can easily slip into detraction, and, as you delight in the attention of your hearer, even into calumny. It's no surprise that many of the early Fathers of the Church regarded gossiping and backbiting to be among the most common, and yet most dangerous, of sins.