Libel generally means publishing or writing a defamatory statement about someone you know is not true (as opposed to "slander," which is an oral statement) [source: Nolo]. But there is also something called "group libel."
In 1950, the state of Illinois prosecuted Joseph Beauharnais for "group libel" — specifically, for defaming African-Americans living in Illinois. (The defendant was arrested for distributing leaflets that asked Chicago government to "halt the further encroachment, harassment and invasion of white people ... by the Negro" [source: Oyez].)
The U.S. Supreme Court ruled in 1952 (Beauharnais v. Illinois) that his conviction was lawful, because you can't make hateful statements about racial or religious groups unless you can prove what you say is true, and that you're saying these things with "good motives" and for "justifiable ends." This ruling became known as the group libel law. As the years passed, however, legal experts deemed the group libel law a poor law. The Supreme Court never repealed the group libel law, yet passed various restrictions to it [source: Volokh].
For example, its Garrison v. Louisiana (1964) ruling basically said it's not unconstitutional to convict someone of libel, but if you do so you must prove the person acted with malice and that the person made libelous statements "with knowledge of their falsity or with reckless disregard of whether they were false or not."More recently, in 1992, the Court ruled unanimously in R.A.V. v. City of St. Paulthat you can't single out bigoted speech as unconstitutional and illegal [sources: Volokh, Oyez, Lisby].