This is a shortened defintition of the ad hominem fallacy taken from the Skeptics Dictionary,
Ad hominem is Latin for "to the man." The ad hominem fallacy occurs when one asserts that somebody's claim is wrong because of something about the person making the claim. The ad hominem fallacy is often confused with the legitimate provision of evidence that a person is not to be trusted. Calling into question the reliability of a witness is relevant when the issue is whether to trust the witness. It is irrelevant, however, to call into question the reliability or morality or anything else about a person when the issue is whether that person's reasons for making a claim are good enough reasons to support the claim [emphasis mine].
... Rather than criticize a person’s premises or reasoning, one asserts something about the person’s character, associations, occupation, hobbies, motives, mental health, likes or dislikes.
I was reading a post on Godless Girl today and it made me think about how common ad hominem fallacies are.