Generally, a personal attack is committed when a person substitutes abusive remarks for evidence when examining another person's claims or comments. It is considered a personal attack when a person starts referencing a supposed flaw or weakness in an individual's personality, beliefs, lifestyle, convictions or principles, and use it as a debate tactic or as a means of avoiding discussion of the relevance or truthfulness the person's statement. It works on the reasoning that, by discrediting the source of a logical argument, namely the person making it, the argument itself can be weakened.
This line of "reasoning" is fallacious because the attack is directed at the person making the claim and not the claim itself. The truth value of a claim is independent of the person making the claim. No matter how morally repugnant a person might be, he or she can still make true claims. For example, a defense attorney may claim that a witness' testimony cannot be trusted because he is a convicted felon. On the other hand, illuminating real character flaws and inconsistencies in the position of an opponent are a vital part of the public political process and of the adversarial judicial process. Use of a personal attack in a logical argument constitutes a formal fallacy called ad hominem, a term that comes from a Latin phrase meaning "toward the man".
That definition is from Wikipedia.
