To win a defamation case, a plaintiff must show four things: 1) a false statement purporting to be fact; 2) publication or communication of that statement to a third person; 3) fault; and 4) some harm caused to the person or entity who is the subject of the statement.
Since the U.S. Supreme Court's 1964 decision in New York Times v. Sullivan, defamation claims have been limited by First Amendment concerns. Thus, for instance, public officials and public figure plaintiffs (people who are famous) must establish the existence of actual malice to recover in an action for defamation. Actual malice means that a statement was made with knowledge that it was false or with reckless disregard of whether or not it was false. In addition, a plaintiff must show actual malice by "clear and convicing" evidence rather than the usual burden of proof in a civil case, preponderance of the evidence.
A private person suing about a matter of private concern need only show negligence, meaning that the defendant knew the statement was false, or would have known if she or he had exercised reasonable care.