Here's a short outline of libel/slander (i.e., defamation) law in
the USA and (generally) in common-law countries.  Some (e.g., UK) 
have moved to stricter requirements on speakers, and no doubt there are
significant variations.


A defamatory statement is one that injures the reputation of another
party, either by lowering the standing of the injured party in the
community or by tending to make others refrain from associating
with him.  However, it's not just any such statement.

-- If the speaker can prove that the statement is true, it is not
   defamatory.
-- For slander, it must _actually_ have had that effect on listeners, 
   not just potentially.  That is, if you orally claim that X is a 
   child-molester and all other tests are met, it's not defamation 
   unless people actually believed your claim.  The assumption of
   actual damage is only assumed in certain specific cases of 
   "slander per se" (as always, assuming the claim is untrue):
        -- charges that plaintiff committed a criminal offence
        -- charges that plaintiff has one of certain "loathsome" 
           contagious diseases
        -- charges that impute unchastity or adultery to any woman
        -- charges disparaging plaintiff's conduct of office, 
           trade, calling, or business being conducted at that time 
-- For libel, actual damages are presumed if the statement is one
   one of a small number of types considered to be "libel per se"
   (defamation that is apparent on the face of a communication):
   charges of criminal activity, adultery, "contagious distemper,"
   or dishonesty, as well as any charge which injures the plaintiff
   in his or her trade, business, or profession."  Otherwise, actual
   damages must be shown by specific and unambiguous evidence of
   wrongful harm, which is called "libel per quod" (as opposed to 
   "libel per se").
-- "Publication":  It must be made to a third party.  X telling Y 
   _alone_ that Y is a child-molester does not defame Y.
-- Similarly, if X (above) tells only Y, and _Y_ then tells others,
   then X hasn't defamed Y -- because Y did the publishing.
-- The statement's not defamation if it's in a situation subject
   to "absolute privilege".  There is a list of such situations,
   including communications with one's spouse, and testimony in
   court or in legislative or executive governmental hearings.
   (It should be noted that you could evade slander, only to be
   charged with perjury.)
-- The statement's not defamation if it's in one of a variety of
   situations subject to "qualified privilege".  Most involve
   speaking on matters of public or community concern, and must
   be made without malice.  This is related to the press's 1st
   Amendment protection against restrictive legislation (such as 
   libel law: NY Times v. Sullivan, US Supreme Court, 1964.)
-- The defamatory statement must be fairly understood by listeners
   to be a statement of fact, rather than opinion.  Opinions need
   not be fair to be privileged:  Unfair opinions are protected
   by law.  This distinction between allegations of fact and of
   opinion is a grey area that would be decided by the Court.  The
   key question would be whether the purported opinion implies some
   false factual claim, e.g. "I _think_ X is a murderer."
-- In most states, it is not defamation unless the speaker was
   negligent in checking his facts.  If he meets an implied standard
   of reasonable care and was mistaken, then he would be not liable.
-- Republishing someone else's libel/slander can itself be defamatory,
   if the "publisher" had the knowledge and opportunity to not do so.  
   E.g., if a caller defames someone on a KGO talk show, KGO is 
   probably not liable, but would not be protected if it later replayed 
   that segment from tape.
-- The statement's not defamatory if the target's reputation is already
   so unsavoury that no damage was done.



Excellent write-up:  http://www.radford.edu/~wkovarik/class/law/1.5libel.html