Here's an example:

"Rush Limbaugh says that drug dealers should get long prison sentences? He was addicted to pain killers, what a hypocrite. Nothing he says is credible."

What makes it an ad hominem fallacy is that the rebuttal discredits the person making the claim, without addressing the facts of the actual claim itself. You can't really discredit a hypothesis simply by discrediting one person making that claim.

Ad hominem (or 'to the man') is when in a debate, someone attacks their opponent rather than the arguments (s)he is putting forward.

Classic examples may be...

"You're wrong; you're a Socialist."

"I happen to know for a fact that you've had several extra-marital affairs. Why should we listen to anything you have to say on morality?"

"He used to work for Big pharma so everything he says is really just propoganda for the big corporations."

Without wishing to split hairs the above is not strictly true:

Another version of this fallacy is seen a lot in work or politics and is as follows:

Statement "This project was a complete failure and achieved none of its objectives."

Answer "I worked really hard on this project."

The answer does not attack the first person rather it promotes the respondant, however the effect is the same - the argument is not answered.