Richard Brandt, in his essay "Ethical Relativism," distinguishes between three types: 1) descriptive relativism; 2) metaethical relativism; and 3) normative relativism. (In Moser and Carson, Moral Relativism: A Reader)
Descriptive relativism states that "the values, or ethical principles, of individuals conflict in a fundamental way. A special form of this thesis, called "cultural relativism," is that such ethical disagreements often follow cultural lines." (25)
Metaethical relativism "denies that there is always one correct moral evaluation." (26)
Normative relativism "asserts that something is wrong or blameworthy if some person or group - variously defined - thinks it is wrong or blameworthy." (28)
One type of normative relativism is considered to be absurd. Some say that: "If someone thinks that it is right (or wrong) to do A. then it is right (or wrong) to do A." Brandt writes: "This thesis has a rather wide popular acceptance today but is considered absurd by philosophers if it is taken to assert that what someone thinks is right really is right for him. It is held to be absurd because, taken in this way, it implies that there is no point in debating with a person what is right for him to do unless he is in doubt himself; the thesis says that if he believes that A is right, then it is right, at least for him." (28)
I've encountered this kind of thinking in my philosophy classes. "Right" or "wrong" are defined by what one thinks or believes to be right or wrong. If that were true then we're not really talking about ethics but about psychology. If someone believes it is right to kill Jews for fun, then for them it is right for them to kill Jews for fun. Which is, of course, absurd.
(Note: the logic textbook I used is Vaughn's Power of Critical Thinking, which has a section on cultural relativism and its absurd [incoherent] implication that, on cultural relativism, a culture cannot be mistaken and is therefore infallible.)