Trolling is when people go on usenet and say really stupid argumentative things in order to get into an argument. After the first message, they like to sit back and watch the fireworks. A successful troll will only take one message to start a fight that will go on for months and cause many people do get disgusted with the newsgroup and quit forever.

In general, it's not the approved way of behaving, but there are circumstances when it can be useful, such as in order to demonstrate that a group of people are just too concerned about something unimportant. If alt.buttons becomes a wastland when you suggest that men's and women's shirts should have buttons on the same side, it suggests that they're a little too concerned.