Specifically, an ad hominem is where someone uses an irrelevant flaw in your
character as an attempt to discredit your
argument.
Ad hominem:
"You argue that we shouldn't let politicians create dangerous precedent by giving them exceptional power, but you yourself voted for Obama knowing he was creating dangerous precedents!" (Attempting to point out a problem with the
person so that his argument seems weaker.)
Probably not an ad hominem.
"You lying scumbag - you made a claim, and I provided evidence that it was false, and you proceeded to defend the lie by putting out more lies, which I've
also bared with further evidence. And frankly, on top of that, you've been rude as all hell. This conversation isn't worth having if this is the way you are going to behave." (Since the topic of the statement is clearly commentary on the debate itself rather than on the topic being discussed, it is a relevant point, and doesn't even attempt to comment on the actual argument or discredit one's argument by discrediting the arguer. It may be
another logical fallacy - an attempt to bury the debate by going meta - but it's not an ad hominem )
You disagree with a position taken. Rather than counter claim with a specific counter claim, you instead smear my intelligence, by insinuating that I am feeble minded, much like the people who fall for "green energy" schemes that simply cannot achieve energy density parity with existing technologies, out of an ideological self-delusionment.
It isn't an in your face "I think you are an idiot, and find your "spin doctoring" hilarious", but it is the very next best thing, and was stated in the same capacity.
....wut.
I think you may have been the only person to read it that way.