Insults vs. Ad Hominems
Brian Leiter has this useful post up about the difference between an unkind remark and an ad hominem attack:
Many names have descriptive and referential content: this goes for “criminal,” “moral cretin,&^rdquo; “moron,” and “liar.” Many of these names (e.g., “moral cretin”) can, of course, also be used metaphorically, though so used, they still have cognitive content, and the individuals to whom the names are attached either do or do not satisfy the descriptive content of the name metaphorically used.
The use of “names” is not an “ad hominem.”
An “ad hominem” is a kind of argument, that is fallacious (though, in some contexts, may actually be fairly reliable: more on that in a moment). The argument has the following structure: X asserts Y; you attack X to undermine Y, e.g., you argue that because X is a certain kind of person, Y is false and/or ought not to be believed. (Note: the fallacy, strictly speaking, would be to conclude from facts about X that Y is false; concluding that Y ought not to be believed based on an attack on X can be reasonable, a point to which we'll return.)