An ad hominem fallacy is when you use a personal attack on someone during a debate instead of attacking the argument. Something to the effect of when you put forth a solid argument, well thought out and presented perfectly. Your opponent retorts with something to the effect of “how can you trust someone who smoked pot in college?” It does nothing to address the actual argument but will make a person listening to the debate think lesser of the one being attacked.
I see this often in debates between theists and atheists from both sides. Usually from the theists because they’ve been backed into a corner and can’t find a logical way out. From the atheist side, usually a neck-beard trying to start a fight (yes I realize the irony of that statement). But in all honesty I think that putting down these sorts of attacks is not a productive way to go about debating. I think its ok to make an attempt to see the point of view from the other side of a debate. I think this makes for a better dialog. Also you can understand it with out accepting it.