An ad hominem argument attacks a person rather than his arguments.
Ad hominem circumstantial points out that someone is in circumstances such that they are disposed to take a particular position (e.g. "You oppose the age of consent because you are a pedophile"). Ad hominem circumstantial constitutes an attack on the bias of a source. This is fallacious because a disposition to make a certain argument does not make the argument false.
An anonymous proverb states, "Great minds discuss ideas, average minds discuss events, small minds discuss people."