An "ad hominem" argument is an argument that is based on who said something (declaring his unworthiness or incompetence etc.) instead of what was said, i.e. the failure to respond to the content of the statements made.
Let me illustrate, what ad hominem means:
Case 1. XYZ is a no good lying hypocritical piece of sewrage. He accused me of posting binaries, when I have done no such thing; and XYZ's posting signature includes a 4Kbyte jpeg. Analysis: Remarks are over the top, but not ad hominem, since a real case is made based not on who said something but based on what was said. Case 2. Dr. XYZ has recently lost his job and I sympathize with his position. Clearly he is upset, and his comments should not be taken seriously. Analysis: Ad hominem. Compassionate, and possible even constructive. But ad hominem, since an argument is dismissed based on who said it, without reference to content. Case 3: I understand the motivations of all agnostics, and know why they attack XYZ. Agnostic motivations are impure, and agnostics suck rocks, so the attack is invalid. Besides, they always use argument ad hominem. Analysis: Ad hominem. Clear and blatant.
Tis helpful illustration was taken from this page.
Answering Islam Home Page