A repfag is somebody who really cares about their reputation online.
Wither it be a Youtube account, facebook, Xbox Live, Steam etc. repfags will take hissy fits if somebody accidentally, or deliberately, uses their account.
Guy 1 - "Hey, I'm just going to go on steam for a moment."
Guy 2 - "NO NO! NOT ON MY ACCOUNT!"
Guy 1 - "Why not?"
Guy 2 - "BECAUSE I HAVE A REPUTATION TO UPHOLD!"
Guy 1 - "You are such a repfag..."