Ad hominem abusive (also called argumentum ad personam) usually and most notoriously involves insulting or belittling one's opponent, but can also involve pointing out factual but ostensibly damning character flaws or actions which are irrelevant to the opponent's argument. This tactic is logically fallacious because insults and even true negative facts about the opponent's personal character have nothing to do with the logical merits of the opponent's arguments or assertions.
Ad hominem as a logical fallacy is when an attack is made in lieu of an argument. However, using an ad-hominem attack after, or in the midst of an argument does not automatically nullify the content within the argument, and does not constitute a logical fallacy, though it may be bad form in a debate.
