Defamation is an intentional false communication that damages a reputation, while libel and slander are types of defamation. Libel is a written communication of this nature, and slander is spoken.
Under United States law, defamation is not a crime but a matter of civil law. For a person to successfully claim damages from defamation, they generally must be considered a private citizen who does not work or voluntarily appear as a public figure. The legal threshold for proving damage from defamation is much higher for a public figure than it is for someone considered a private citizen. The law requires public figures to prove "actual malice," or a clear intent to harm a reputation by knowingly making false statements, in the interest of protecting freedom of speech and the ability to debate controversial issues in the public sphere. Private citizens derive a greater protection under the law as they are not expected to be under public scrutiny and also usually do not have access to the communications channels that public figures do.
Actual requirements for a defamation case also vary between states. General rules are that the statement must either be published or have been heard by a third party. The statement must also be provably false and injurious.