When you leave your computer account (usually at work) unlocked, someone finds it and they send emails out (usually to a large amount of people) pretending to be you but saying ridiculous or embarrassing things.
A really good buttshark is one that people can't tell if the person actually sent it or if they got buttsharked. Another variation is to email one person, usually that person's mortal enemy, and apologize for everything and ask them out for beers but BCC everyone else.
Oh come on, you know that John got buttsharked. He has a girlfriend!