Defamation refers to a false statement made against another party, which is published, broadcast or otherwise circulated to third parties. It is not necessary to say "defamation of character" -- you can only defame one's character or reputation. It is important to note that the statement must be false (truth is a defense!). If you say that someone is suffering from AIDS, and he considers that defamatory, he cannot win the case if he actually suffers from AIDS. If you say something defamatory to a person but nobody else hears it, it won't be defamatory because it must be transmitted to one other person.
Do not confuse "libel" with "liable." Libel refers to written published defamation. So, if a newspaper defames you in writing, the newspaper is guilty of "libel." The newspaper story is thus "libelous." Of course, when the newspaper libels you, it may be "liable" to you for damages. Libel is the written form of defamation; liable means "legally responsible."
Slander is the spoken form of defamation. If I stand up in the park and call you an "unethical businessman," I may have slandered you (assuming the statement is not true). At common law, a person was guilty of slander per se by accusing a person of having a loathsome disease, or stating that they stunk in their business or trade, or accusing a woman of being unchaste. In modern times, broadcast stations can be sued for slander as well as libel (if they keep recorded, permanent tapes of the spoken defamation).