A troll is someone who posts controversial, inflammatory, irrelevant or off-topic messages in an online community. Their intent: to provoke others. A troll wants to take your blog, twitter, social network conversation and to wreck it.
An original troll: our fellow beneath the bridge. Three Bill Goats Gruff. A Norwegian tale many of us know: De tre bukkene Bruse. If you don't know this one. Find it for yourself.