A troll is a post written just to provoke a response, or the person who makes such a post.
A prime example would be posting "Python is so much better than Perl!" It wouldn't matter whether the poster really believed that; he would write it because he knows it'll stir things up.
Someone who responds to such a post is said to have been trolled.
It's a dud node, often but not necessarily posted by Anonymous Monk. It may contain some insulting, worthless, or obviously stupid information- like that recent British cheese poll.