There is a more sympathetic approach to ad hominem that is found is other approaches to informal logic. This approach accepts that arguments can be a form of reasoning which is acceptable in principle. For instance, if one were debating whether a political candidate should be elected for the Senate, an ad hominem argument can be broached that the candidate does not have sufficient experience in politics for the position.
Still another approach to ad hominem arguments allows ad hominum reasoning as rhetorical devices. That is, they are understood in terms of Aristotle's suggestion that the ethos [the high moral impression] of a speaker plays a crucial role in determining whether an argument is persuasive or not. An example of this type of ad hominum exists in Patrick Henry's famous lines
I know not what course others may take; but as for me, give me liberty, or give me death!
An ad hominem argument is one in which you attack the person who is making an argument that you do not like instead of attacking the argument itself.
An example of this would be, for example, if you do not like the health care bill that the President just signed today. You could say something like "President Obama, who shouldn't be president anyway because he was not born in America, is in favor of this bill. We must oppose it in any way we can."
That is an ad hominem argument. It is fallacious because it is not based on "here's why the bill is bad." Instead it is saying "I don't like Obama, and therefore the bill is bad."