An ad hominem argument is one in which the person who is being argued against is directly targeted rather than their points. In other words if I say there is no evidence that UFOs are alien craft and you say I’m a narrow-minded egotistical moron, that’s an ad hominem. You can see how the point being made was completely ignored in favour of a personal attack on the character of the person making it. Unfortunately many discussions tend to devolve into a back and forth exchange of personal attacks like the one illustrated rather than focussing on the actual facts and premises of the discussion.
One reason that this happens is that people become so invested in the ideas and beliefs that they hold that any contradiction to them itself feels like a personal attack, in this case it would seem perfectly natural to return the salvo. Another reason may be a phenomenon known as Cognitive Dissonance, whereby if a person’s views are contradicted by conflicting evidence this can create a feeling of discomfort within the individual, which requires relief. The easiest way to do so is to discredit, at least in that person’s mind, the source of the conflicting evidence.
Whether it comes about through feelings of personal affront or esoteric interactions within the psyche the ad hominem is generally considered to be the argumentative low road and should be avoided. If a debate cannot be won through the logical points made on the strength of the facts then the resolution should be deferred until such time as there is more information available. Ok, that’s too much to ask, but keep it clean and above the belt.