Ad Hominem
The Fallacy
Attacking your opponent's character or personal traits in an attempt to undermine their argument.
Why it's wrong
Ad hominem attacks can take the form of overtly attacking somebody, or more subtly casting doubt on their character or personal attributes as a way to discredit their argument. The result of an ad hominem attack can be to undermine someone's case without actually having to engage with it.
Example
After Sally presents an eloquent and compelling case for a more equitable taxation system, Sam asks the audience whether we should believe anything from a woman who isn't married, was once arrested, and smells a bit weird.