The arguemtentum ad hominem is an informal fallacy. It is the assertion that a given argument is valid or sound (or invalid or unsound) for no other reason than the qualities of the person making it.
It is a principle of logic that arguments are valid or invalid based only on their forms. That is to say, that a formal argument is valid if and only if (iff) for all arguments of this from, true premises can never lead to a false conclusion. Validity is a quality contingent only on form. It does not matter who formulates it, how, or why. A sound argument is a formally valid argument that has true premises, and -- by virtue of being a formally valid -- a conclusion that must therefore also be true.
Let's take 'X=2+2'. therefore 'X=4'. A very popular argument, generally considered arithmetically valid and sound (ignoring for a moment Bertrand Russell). If we accept this argument as valid and sound, it is valid and sound no matter who says it: it's true if formulated by a white male. Or a black man. Or a Jew. Or a child. Or a mass murderer. Or someone developmentally disabled. Or Lucifer himself. To assert otherwise would be to make an argument ad hominem.
Or we can take an unsound argument: 'X=2+2' and 'X=5'. This is not a valid or sound arguement, no matter who says it. Being formulated by a priest, or a politician, or a media pundit, or a doctor of philosophy, or Yeshua ben Yosef cannot make this a valid or sound argument. To assert otherwise is again, an argument ad hominem.
Now of course, this treads dangerously close to the concept of 'appeal to authority'. Appealing to an authority is also technically an informal fallacy of the form ad hominem. The only realm where appeal to authority can be a successful argument is when it can be strongly asserted that the authority in question has a specific body of knowledge, training or understanding by which such authority can make decisions about the truth or falsity of assertions that canot be made by someone lacking that knowledge, training, or understanding. That is to say, I can appeal to authority if it can be shown that if I were to undertake the training necessary in that field and gained for myself the expert knowledge and understanding in question, I would be capable of making the exact same determinations.
Googlebombing for a cause: www.minnesotangos.org