An example of this would be to accuse a supporter of Marriage Equality of being communist. Even if the claim was true, being a communist has nothing to do with the argument that everyone should have the right to get married. This is an attempt to poison the well against the argument by linking the issue to the character of the person and then destroying said persons character.
ad hominem (ad ho-mi-nem) \ad Hom-uh-num, -nem, ahd-\, adjective;
1. Appealing to feelings or prejudices rather than intellect.
2. Marked by or being an attack on an opponent's character rather than by an answer to the contentions made.
Hopefully this tip will help you construct a much strong argument in your writing. Instead of making using an ad hominem, personal attack, you can now focus your attention on meat and potatoes of your opponent's argument. Make sure to point out to your friends when they making this logical fallacy, and please leave us an example of an ad hominem attack. Thank you.