Ad Hominem
The Fallacy
Calling someone a mean name instead of talking about their idea.
Why it's wrong
When you can't think of a good reason why someone's idea is wrong, you just attack them personally. It's not nice and it doesn't prove anything.
Example
I don't care if Sarah says the sky is blue. She is a big poopy-head, so she must be wrong.
What the Fallacy?! (ELI5 Edition)