According to the Associated Press Stylebook and Libel Manual:

"Libel is injury to reputation."

"Words, pictures or cartoons that expose a person to public hatred, shame, disgrace or ridicule, or induce an ill opinion of a person are libelous."

So, right off the bat, you don't have to worry about libel if your bumper sticker is just an idea, say "Free the Whales." On the other hand, "Joe Blow is a whale killer" could get you in trouble. The courts have also held that it's impossible to libel a corporation. And, as a general rule, you can't libel a whole class of people -- say Christians. But if you say "Disciples of the First Church of Whoosenits on Lincoln Avenue are a bunch of goat-lickers," and it so happens that the First Church has only three members, whom everyone knows by name in your sleepy little burg of 1,200.... Well, the courts have held that could indeed be libel.

Thorough understanding of the Supreme Court cases New York Times v. Sullivan; Gertz v. Robert Welch; and Milkovich v. Lorain is a good place to start, but by no means finish, an understanding of U.S.libel law.

Truth is a defense against libel, though not against public disclosure of private facts. If Joe Blow is in fact a whalekiller, it ain't libel. Statements of pure opinion are never libel either, although opinion drawn from "false facts" is excepted. As Chief Justice William Rehnquist writes in Milkovich:

If a speaker says "In my opinion John Jones is a liar," he implies knowledge of facts which lead to the conclusion that Jones told an untruth. Even if the speaker states the facts upon which he bases his opinion, if those facts are either incorrect or incomplete, or if his asssement of them is erroneous, the statement may still imply a false assertion of fact. Simply couching such statements in terms of opinion does not dispel these implications; and the statement, "In my opinion Jones is a liar," can cause as much damage to reputation as the statement "Jones is a liar."

The constitutional guarantees require, we think, a federal rule that prohibits a public official from recovering damages for a defamatory falsehood relating to his official conduct (our emphasis) unless he proves that the statement was made with "actual malice" -- that is, with knowledge that it was false or with reckless disregard of whether it was false or not.

Hypothetically, it may be possible for someone to become a public figure through no purposeful action of his own, but the instances of truly involuntary public figures must be exceedingly rare. For the most part, those who attain this status have assumed roles of especial prominence in the affairs of society. Some occupy positions of such persuasive power and influence that they are deemed public figures for all purposes. More commonly, those classified as public figures have thrust themselves to the forefront of particular public controversies in order to influence the resolution of the issues involved. In either event, they invite attention and comment.

Even if the foregoing generalities do not obtain in every instance, the communications media are entitled to act on the assumption that public officials and public figures have voluntarily exposed themselves to increased risk of defamatory falsehoods concerning them. No such assumption is justified with respect to a private individual. He has not accepted a public office or assumed an "influential role in ordering society." He has relinquished no part of his interest in the protection of his own good name, and consequently he has a more compelling call on the courts for redress of injury inflicted by defamatory falsehood. Thus, private individuals are not only more vulnerable to injury than public officials and public figures; they are more deserving of recovery.

Skeletons, we've all got 'em: Stuff people could say about us that may not be fales but could be seen as somewhat twisted, kinky or embarrassing. Stuff we may not even tell our girlfriends, or husbands about. (OK, maybe not all of us have this stuff. But if you don't: You're probably boring).

Occasionally, maybe after a couple of drinks, we let these things slip. And then we know that somebody out there's got the goods on us. Disseminating these "goods" -- personal information that a reasonable person would find to be highly offensive and not of legitimate public concern -- is public disclosure of private facts.

There isn't the body of judicial opinion governing this area of law that there is for libel, but 38 states recognize disclosure of private facts as an actionable tort. California courts held in 1983, for example, that a newspaper column commenting on a college-student-body leader's sex change operation was a disclosure of an embarrassing fact.

The important thing to note is, truth is not a defense here. In fact it's the central ingredient to the offense. If you think you're clever because you know some salacious tidbit about somebody else.... You're not.