Defamation—also called calumny, vilification, traducement, slander (for transitory statements), and libel (for written, broadcast, or otherwise published words)—is the communication of a statement that makes a claim, expressly stated or implied to be factual, that may give an individual, business, product, group, government, or nation a negative image. This can be also any disparaging statement made by one person about another, which is communicated or published, whether true or false, depending on legal state. In Common Law it is usually a requirement that this claim be false and that the publication is communicated to someone other than the person defamed (the claimant). (from Wikipedia)
Where we run into trouble with defamation laws on the Internet is, one man’s defamation is another person’s First Amendment-protected free speech. Criticism/Parody/Critique can look like defamation or cyberbullying, so a lot of it kinda overlaps with the rest.
With defamation the statement generally has to be false and then you have to prove damages (except in the case of defaming a woman’s virtue!). However, parodies and critical humor are often ridiculous in the first place, but could also damage reputations. Cyberbullying is just mean, but depending on the platform (ahem, I’m talking about YOU, FORCHAN) saying shocking things could just be the culture. Plus, the First Amendment guarantees freedom of speech, so this whole thing gets very fuzzy.