1. Person A makes claim X.
2. Person B makes an attack on person A.
3. Therefore A's claim is false.
Example of Ad Hominem:
Robert Spencer: "I believe that islam is morally wrong."
Asmodeus: "Of course you would say that, you're a jew."
Robert Spencer: "What about the arguments I gave to support my position?"
Asmodeus: "Those don't count. Like I said, you're a jew, so you have to say that islam is wrong. Further, you are just a lackey to Israel, so I can't believe what you say."