Libel is published (written) defamation which tends to injure a person's reputation. It is contrasted and occasionally accused 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 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, if it is the truth, the defendant 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 web sites 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.