I've heard people use the terms libel and slander interchangeably, but I know that they are different in the eyes of the law. What are the defining characteristics of each?
Libel is the use of false, defamatory claims about someone in written or printed form. Slander likewise denotes false statements that damage a person’s reputation, but it is committed orally or in any other transient form
So a false claim satisfying the definition of defamation would be libel if written (including on the Internet, per Varian Medical Systems, Inc. v. Delfino), and slander if made in a transient form.