Libel is published (written) defamation which tends to injure a person's reputation. It is contrasted and occasionally confused with slander, which is spoken defamation. Within the broad category of libel, there are two types, libel per se and libel per quod.
One of the defenses against the charge of libel is truth. In the United States, the truth is always an absolute defense. This means that regardless of how nasty or vicious the statement appears (for example, malicious detraction), if it is the truth, the defendant in a libel suit will always prevail. For a court to rule otherwise would be prima facie evidence of censorship, and overturned on appeal.
Furthermore, the statement must be presented as a fact. Parody or jokes are not considered defamatory because they are not intended to be taken seriously. However, this defense will fail if a reasonable person would consider the statement to be factual, i.e. from the credentials of the source.
Under Section 230 of the Communications Decency Act, web hosts enjoy a limited immunity from civil prosecution. The reasoning behind the law is that the multitude of websites per hosts makes it impossible for them to know what exactly is being hosted. This immunity usually extends to libel, with certain exceptions. A webhost made aware of defamatory statements must immediately remove them to remain immune. Also, statements made by or with the knowledge of the hosts are fair game for a lawsuit. One of the more notable uses of Section 230 is by the Wikimedia Foundation, which has a unique position as both webhost and website. The Foundation has always prevailed, due to the sheer amount of data hosted and the relative autonomy of each site. Their success has led to some websites believing such laws do not apply to them. However, these sites are severely mistaken because they are not webhosts, but rather individual sites. Unlike the Foundation, which serves as a confederation of websites, the owners of other websites are in direct control of their content, thereby incurring liability for it.
Examples of Libel
Answers in Genesis, Living Waters, Creation Ministries International, the Institute for Creation Research, Richard Fangrad, Calvin Smith, Ken Ham, Ray Comfort, the Bible, the Creation Museum, the Ark Encounter, Got Questions Ministries, and several others have all been libeled; with Wikipedia (and HuffPost and "Rational"Wiki) accusing Ham and his affiliates of pseudoscience, Media Bias/"Fact Check" accusing CMI, Fangrad, and Smith of the same thing, when in reality they are doing the exact opposite of pseudoscience. PewDiePie has been accused by "Rational"Wiki (quotation marks indicate not really) of encouraging his followers to hack printers to spread messages telling people to subscribe to him, when in reality the hacking started entirely without PewDiePie knowing that it happened.