A recent example of an ad hominem argument.
After being provided Neil deGrasse Tyson's "Perimeter of Ignorance" paper (see footnote) which puts Newton, Einstein, et al's religious sentiments into perspective, instead of critiquing the paper a believer went off questioning NDT's "smartness": "Did he ever win a Nobel Prize? Please elaborate why [we] would think that he is smarter or wiser than Albert Einstein or Sir Isaac Newton." And even after being told that "an idea stands and falls on its merit, not from whom it came from," this believer forges on, none the wiser, and hauls out NDT's curriculum vitae and resume from wikipedia, placing emphasis on NDT's being "a frequent Guest on the Game Show called Jeopardy," finally concluding "I am sorry but his greatest 'Achievements' are certainly nothing compared to the 'Achievements' of Albert Einstein, and Sir Isaac Newton. Are you kidding Sir???"
As to whether this person even bothered to read NDT's essay is unclear. He certainly does not even broach the ideas contained therein.
What is an ad hominem? It consists of "attacking the person instead of attacking his argument" [link]. "This tactic is logically fallacious because insults and even true negative facts about the opponent's personal character have nothing to do with the logical merits of the opponent's arguments or assertions" [link].
An ad hominem argument has the following basic form:
1. Person A makes claim X.
2. Person B makes an attack on person A.
3. Therefore A's claim is false.
Person A corresponds to NDT and person B to the believer.
One crucial question is whether B would've said what he did if NDT's essay had been, say, entitled "The Perimeter of Enlightenment" and had hailed Newton et al. as being warranted in invoking an intelligent designer for those mysteries which at the time eluded scientific explanation. Well apparently not, since when confronted with this very question, B simply refused to answer.
When you think about it, science would never advance and would become dogmatic and a cult worshiping the pronouncements of a few if the ad hominem rule were to prevail. Young scientists and thinkers would never get off the ground since they have yet to make their mark in history by adding to the pool of knowledge or even overthrowing wrong ideas by their esteemed predecessors. So when their ideas are at odds with the status quo they would be compared with the giants who've come before them and since their CV and resume are practically blank they would be pooh-poohed and never given an audience. Their ideas would be trampled upon and deemed heretical simply because they have no Nobels or awards to show off, no celebrity status, no biographies, no books written about them, none of the "blings" to show they're smarter and oh so important.
And rather obviously, scientists win Nobel prizes after their ideas have proved to be groundbreaking and shown to be true, and not because they scored 300 on their IQ test or because their CV is a hundred pages thick or that they didn't frequent game shows. Had we applied the above believer's criteria for judging ideas we would've said of Einstein when he came up with his relativity theory (thus supplanting Newtonian mechanics) at the beginning of the 20th century: "Einstein was a patent office clerk! Are you kidding me?! Has he won a Nobel Prize? Please elaborate why we should think he's smarter or wiser than Sir Isaac Newton?"
Far more entertaining than the paper is Tyson's Beyond Belief 2006 talk with the same title: Part 1 Part 2 Part 3 Part 4