The ad hominem logical fallacy, in Rugby terms, is playing the player rather than the ball. (If you have no problem with this, consider the ball to be elsewhere completely.)
An Ad Hominem is a general category of fallacies in which a claim or argument is rejected on the basis of some irrelevant fact about the author of or the person presenting the claim or argument.
This post does not contain an in-depth discussion of the ad hominem fallacy, so follow some of the links provided if you need to know more.
The ad hominem fallacy fallacy is a particularly educational page. It provides an in-depth explanation when something is not an ad hominem attack, through use of many examples. I think this is a must-read. Skeptico has a good page dealing with Ad hominem attacks directed at him, explaining neatly what the ad hominem attackers are supposed to do instead of using absurd personal attacks. Then, of course, there is always the Wikipedia ad hominem page.
The first example of an ad hominem attack directed at me, that I can remember, was by Johan Kruger. He is a member of Creation Ministries International. He wrote the following in his first comment on my blog:
However, from several less than honest subsequent statements on your blog, one wonders if you really meant it. So, can folk really trust ANYTHING you say on your blog?
I have no clue what he is referring to with “several less than honest statements”. In my opinion he did not identify any dishonest statements. I take honesty extremely seriously, and would like to be informed if anyone notices anything they consider dishonest, as I would then like to rectify the matter. Instead of helping me improve my arguments, Johan Kruger prefers to make a sweeping statement in an apparent attempt to undermine everything I say on my blog. I’m quite certain this is an ad hominem attack?