Simply put, any act wherein an individual or entity make a false claim/s about another individual or entity and damages their reputation amounts to defamation. Basically, there are two forms of defamation: slander and libel. Even though they are two different concepts, the striking similarity in them can leave you confused. In legal terminology, defamation is defined as a false accusation or a misinterpretation of someone's action or words, which eventually hampers the other person's reputation.
In a broad sense, the term covers any defamation that can be categorized as transitory, instead of permanent. For instance, in a public meeting, a person falsely accusing his rival of serving a prison term for a felony would amount to slander―irrespective of whether his reputation is already damaged or there are chances that it can be damaged due to this accusation.
As per slander laws, the onus is on the person who files the defamation lawsuit to prove that the said statement or gesture led to defamation of character, and/or eventually resulted in financial loss for him. That being quite difficult to prove, most of the cases of slander don't go to the court of law.
This form of defamation most often covers written material or published pictures which defame the individual or entity. An apt example of this would be a soccer player, in his autobiography, falsely accusing his previous club of fixing a game. This would amount to libel, as it defames the reputation of the said club.
Libel is relatively easier to prove in the court of law, as the victim has enough evidence to prove that he has suffered damage to his reputation, which has to be compensated, most often, in terms of financial loss.
After going through both the concepts, you will realize that the most prominent difference between the two is the fact that the former is a case of verbal (oral) defamation, while the latter is a case of published (written) defamation.
Secondly, the lawsuits differ in both the cases. In case of slander, the person who has made these statements doesn't have to prove that they are true. Instead, the person suing for slander has to prove that the defamatory statements made against him are false. In contrast, when it comes to libel, the complainant has proper evidence to prove defamation and seek a compensation for the same.
In cases of slander, the person being charged with defamation can argue that he didn't defame the complainant intentionally. Lack of evidence also makes the case weak for the complainant. If the person being charged for defamation is a prominent public figure, then the task becomes even more difficult, as the complainant doesn't just have to prove that the accusations made against him are false, but also prove that the person who defamed him did it intentionally.
Going by the guidelines set by the Supreme Court of the United States, only factual interpretation, and not a person's opinion, can be considered a slander or libel. The freedom of expression is no doubt one of the fundamental human rights in democracy, but one has to ensure that he doesn't use it to malign other people's reputation.
In the end, defamation―regardless of whether it is slander or libel―is an act which doesn't fit into the legal or moral framework of the society.