The Internet Tough Guy is a feature in all Internet social forums. These are people who poison discussions with anger, hatred, and threats. Some are malicious. Some are crazy. Some are just afflicted with a rotten sense of humor. Whatever their motives, they're a scourge. It takes precious little trolling to sour a message-board. A "troll" -- someone who comes onto an online community looking to pick fights -- has two victory conditions: Either everyone ends up talking about him, or no one talks at all. And where two or more trolls gather, they'll egg each other on, seeing who can anger and disrupt the regular message-board posters the most.
It can be distressing. If you're part of a nice little community of hamster-fanciers, Trekkers, or Volkswagen enthusiasts, it's easy to slip into a kind of camaraderie, a social setting in which everyone talks about life, aspirations, family problems, personal triumphs. In some ways, it doesn't matter what brought you together -- the fact that you're together is what matters.
Then, almost without warning, your community goes toxic. Someone in your group undergoes a radical personality shift and begins picking fights, or someone new comes to the party with an agenda. Or, worst of all: Your little clubhouse achieves some small measure of fame and is overrun by newcomers who don't know that Liza is a little bit touchy on the subject of hamster balls, or that old Fred gets into a froth anytime someone asks about retrofitting a bud vase into a vintage Beetle, or that everyone here actually kind of knows Wil Wheaton from reading his blog and he's a total mensch, so jokes about shoving Wesley out the airlock are frowned upon.
Sometimes, you rebound. More often, you tumble. Things get worse. The crowds get bigger, the fights get hotter. Pathologically angry (but often funny) people show up and challenge each other to new levels of vitriol.
In extreme cases, you end up with the kind of notorious mess that Kathy Sierra found herself in, in which trolls directed such bilious, threatening noise towards a harmless advocate for "passionate users" in web-applications that she withdrew from speaking at O'Reilly's Emerging Tech conference.
You can deal with trolls in many ways. Many trolls are perfectly nice in real life -- sometimes, just calling them on the phone and confronting them with the human being at the other end of their attacks is enough to sober them up. But it doesn't always work: I remember one time I challenged someone who'd been sending me hate mail to call me up and say the words aloud: the phone rang a moment later and the first words out of my troll's mouth were, "You f*cking hypocrite!" The conversation declined from there.
Trolls can infect a small group, but they really shine in big forums.