A personal attack is committed when a person substitutes abusive remarks for evidence when attacking another person's claim or claims. 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. After all, no matter how repugnant an individual might be, he or she can still make true claims.
Not all ad Hominems are fallacious. In some cases, an individual's characteristics can have a bearing on the question of the veracity of her claims. For example, if someone is shown to be a pathological liar, then what he says can be considered to be unreliable. However, such attacks are weak, since even pathological liars might speak the truth on occasion.
In general, it is best to focus one's attention on the content of the claim and not on who made the claim. It is the content that determines the truth of the claim and not the characteristics of the person making the claim.
1. In a school debate, Bill claims that the President's economic plan is unrealistic. His opponent, a professor, retorts by saying "the freshman has his facts wrong."
2. "This theory about a potential cure for cancer has been introduced by a doctor who is a known lesbian feminist. I don't see why we should extend an invitation for her to speak at the World Conference on Cancer."
3. "Bill says that we should give tax breaks to companies. But he is untrustworthy, so it must be wrong to do that."
4. "That claim cannot be true. Dave believes it, and we know how morally repulsive he is."
5. "Bill claims that Jane would be a good treasurer. However I find Bill's behavior offensive, so I'm not going to vote for Jill."
6. "Jane says that drug use is morally wrong, but she is just a goody-two shoes Christian, so we don't have to listen to her."
7. Bill: "I don't think it is a good idea to cut social programs."
Jill: "Why not?"
Bill: "Well, many people do not get a fair start in life and hence need some help. After all, some people have wealthy parents and have it fairly easy. Others are born into poverty and..."
Jill: "You just say that stuff because you have a soft heart and an equally soft head."