And why do people do it?
The word comes from a Norse monster but the troll is a very modern menace.
Trolling is a phenomenon that has swept across websites in recent years. Online forums, Facebook pages and newspaper comment forms are bombarded with insults, provocations or threats. Supporters argue it's about humour, mischief and freedom of speech. But for many, the ferocity and personal nature of the abuse verges on hate speech.
In its most extreme form it is a criminal offence. Last year, Sean Duffy was jailed for 18 weeks after posting offensive messages and videos on tribute pages about young people who had died. One of those he targeted was 15-year-old Natasha MacBryde, who had been killed by a train. 'I fell asleep on the track lolz' was one of the messages he left on a Facebook page set up by her family. Colm Coss was jailed for 18 weeks after posting obscene messages on Facebook sites set up in memory of the Jade Goody, the Big Brother star, and several other dead people.
Trolling appears to be part of an international phenomenon that includes cyberbullying. Cyberbullying is when one person or a group of people try to threaten, tease or embarrass someone else by using technology, such as a mobile phone or the internet, and it is just as harmful as bullying in the real world. Trolling is a broad term, taking in everything from a cheeky provocation to violent threats.
A recent suspected victim of trolling and cyberbullying is model Claudia Boerner, who was found dead in her flat in Germany. While the circumstances surrounding her death aren't yet clear, it is widely believed to be connected to the staggering amount of online abuse she received after starring in the German version of TV's Come Dine With Me.
Why people troll continues to baffle the experts.
'Online people feel anonymous and disinhibited,' says Prof Mark Griffiths, director of the International Gaming Research Unit at Nottingham Trent University. 'They lower their emotional guard and in the heat of the moment may troll either reactively or proactively. It is usually carried out by young adult males for amusement, boredom and revenge', he adds. 'And often they'll say something online that they'd never dare to say to your face'.
Perhaps trolls and cyberbullies don't realise the extent of the harm and suffering they can cause. Or perhaps they do, but they just don't care.
What does the law say?
The Communications Act 2003 governs the internet, email, mobile phone calls and text messaging.
Under section 127 of the act it is an offence to send messages that are 'grossly offensive or of an indecent, obscene or menacing character', and the offence occurs whether those targeted actually receive the message or not.
So a good reason to think twice if you're thinking of trolling!
A few other things to consider....