Trolling is on-line bulling.
A Troll (or Internet Toll) is a person who posts inflammatory, or inappropriate content to disrupt a normal on-topic discussion in, say a newsgroup, chat room, or in the comment section of a blog. Mostly they do it for their own amusement.
As trolling is increasingly taking on a harassment it has lead to people to closing their social media accounts and worse. Also see: Shitstorm and doxing.