Image caption Celebrities are often targets for abuse

An internet "troll" has been jailed for mocking dead teenagers on various websites. Public figures, including Stephen Fry and Miranda Hart, have also been victims of trolling. So what is it and why do people do it?

For some the word derives from a fishing term for towing bait behind a boat, for others it comes from the Norse monsters. But today trolling is more likely to involve a keyboard and mouse than a trawler, and if not a monster, it is a very modern menace.

Opponents might characterise it as the internet equivalent of road rage, vandalising a grave, or kicking a man when he's down.

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. On Tuesday 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.

High-profile cases Natasha MacBryde - Sean Duffy was jailed for 18 weeks for posts on social networking sites about the 15-year-old after she took her own life

Hayley Bates - MP Karen Bradley raised trolling in Parliament after a Facebook page was set up mocking the 17-year-old's death in a car crash

Jade Goody - Colm Coss was jailed for 18 weeks after posting obscene messages on Facebook sites set up in memory of the Big Brother star and several other dead people

Duffy is the second person to be jailed for trolling in the UK. Last year Colm Coss was imprisoned for posting obscene messages on Facebook tribute sites, including that of Jade Goody.

Trolling appears to be part of an international phenomenon that includes cyberbullying. One of the first high-profile cases emerged in the US state of Missouri in 2006, when 13-year-old Megan Meier killed herself after being bullied online. The bully, Lori Drew, was a middle-aged neighbour who had set up a MySpace account to win - and later betray - her trust. Drew was acquitted of unauthorised computer use in 2009 due to concerns that a conviction would criminalise false online identities.

The First Amendment of the US Constitution protects free speech and makes it difficult to punish people who post offensive messages. But concern over internet vitriol is growing.

Facebook's former marketing director Randi Zuckerberg and Google head Eric Schmidt have both suggested anonymous posting should be phased out.

One of the difficulties is that trolling is a broad term, taking in everything from a cheeky provocation to violent threats. And why people do it 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.

Arthur Cassidy, a social media psychologist, says young people's determination to create an online identity makes them vulnerable to trolling. Secrecy is jettisoned in favour of self-publicity on Facebook, opening the way for ridicule, jealousy and betrayal.

And the need to define themselves through their allegiance to certain celebrities creates a world in which the rich and famous become targets for personal abuse. As a result trolling is "virtually uncontrollable" until the government forces websites to clamp down, he says.

But it's not just young people. Scan any football, music or fan site and there are people of all ages taking part in the most vituperative attacks. But many of the theories that have been put forward as to why people do it don't stand up, says Tom Postnes, professor of social psychology at Groningen University in the Netherlands.

View from the internet forums Will Brooks on setting up Myfootballclub.co.uk It was £35 to join MyFC so I don't think anyone joined with the intention of trolling. But disagreements on the forum all too easily turned to abuse. Finding out that respected professionals in their mid-fifties could post in that way was an eye opener. I've since discovered that forums have a habit of turning sour as it only takes a minority to skew them. As a format they've lost their innocence.

After researching "flaming" - the term for trolling in the early days of the internet - he rejects the idea that people "lose it" when online. If anything they become more attuned to social convention, albeit the specific conventions of the web. Provoking people appears to be the norm in some online communities, he says.

Most trolling is not criminal - it's about having a laugh, says Rob Manuel, co-founder of the website B3ta, which specialises in altering photographs for comic effect. "Trolling taps into people's desire to poke fun, make trouble and cause annoyance," he says.

He first became aware of the phenomenon in the 90s when a friend cross-posted on fan sites for Lord of the Rings and Star Wars, asking: "Who'd win in a fight - the Emperor or Gandalf?" Manuel says his friend sat back and laughed like some "mad scientist looking at insects in a jar" as hundreds of passionate posts followed.

'No guilt'

We're all capable of becoming a troll, says Jaron Lanier, a computer scientist in the US and author of You Are Not A Gadget. Lanier admits he has sometimes behaved badly online and believes the cloak of anonymity can encourage people to react in extreme ways.

"The temptation is there and we can get caught up in impulses. If someone reacts, it's emotional and it can be hard to get out of. We can all become trolls."

Image caption Former Facebook executive Zuckerberg says anonymous posting should be phased out

Twitter has given the public direct access to celebrities. And stars, including Stephen Fry and Miranda Hart, have temporarily left the website after coming under fire. Internet experts say the key is not to "feed the troll" by offering them a response. Comedian Dom Joly takes a different approach.

He describes himself as "troll slayer" and takes pleasure in tracking down the culprits and exposing them to public shame, especially from close family.

"There's something about a bully that really annoys me," he says. "They'll say something online that they'd never dare to say to your face."

The deviousness is "freaky". He discovered that one of those who'd threatened him was a 14-year-old girl with nine different online identities. They aren't always very intelligent about how they do it, he says.

The law 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"

The offence occurs whether those targeted actually receive the message or not Communications Act 2003 - Section 127

"One guy tweeted from his work account that he hoped my kids die of cancer. I let the MD of the firm know and the guy was fired. I felt no guilt, he should have gone to prison."

Some think regulation is needed, but trolling is not the internet's fault, says Jeff Jarvis, author of Public Parts. "The internet does not create special threats. It's a public square where people will be saying all sorts of things, some of them offensive."

The answer is for newspaper websites and online forums to employ sufficient moderators to prevent the comments spiralling into petty vendettas, he says. To ban online anonymity in order to prevent trolling would be to remove the right of whistleblowers and dissidents to get their message across, he adds.

Manuel agrees. "People are saying nasty, stupid things. So deal with it. Shutting down free speech and stamping on people's civil liberties is not a price worth paying."