According to Scandinavian folklore, trolls are grotesque and slow-witted, live in isolation, are uncivilised and like to frighten human beings.

These days, the term ‘troll’ is used for something less mythical – but the description still pretty much fits.

A troll is an online hater, spitting venom via internet forums and social networking site Twitter. When the term first started appearing, it was often applied to people who gatecrashed online memorial pages, posting cruel, offensive comments aimed at causing distress for grieving families.

Now it seems barely a week goes by without another troll hitting the headlines for online abuse.

Singer and X Factor judge Gary Barlow and his wife were recently plagued with cruel jibes on Twitter within days of suffering the tragedy of their daughter being stillborn.

Diver Tom Daley, after a non-medal-winning day at the Olympics, was the target of a Twitter posting that he’d “let down” his father, who died last year, and Team GB weightlifter Zoe Smith and swimmer Rebecca Adlington have both had their appearances mocked by trolls.

And in April, while most people watched in horror when footballer Fabrice Muamba collapsed during a match, somebody was already tweeting ‘jokes’ about him dying. Muamba beat the odds to survive, and the tweeter in question, 21-year-old Liam Stacey, was jailed for 56 days after his actions were deemed racist.

While celebrities and other people in the public eye often turn to Twitter to communicate directly with fans, or ensure they’re not misrepresented, their high-profile status often makes them ‘fair game’ for trolls. But it’s not just the rich and famous who are targeted by online abuse and nastiness.

Shipley teenager Sarah (not her real name) fell victim to online bullies who targeted her through Facebook and Twitter. “It started with a bit of falling out at school, the usual thing for girls their age, then it turned more sinister,” says Sarah’s mother. “Sarah started getting abuse on the internet, mainly attacks on her looks, and she felt she couldn’t escape from it and it was out there for everyone to see.

“Bullying in the playground is one thing, but when they start posting cruel comments online it’s there for the world to see. Sarah was 13, an age when girls are very self-conscious, and she was devastated at what was being written about her.

“Thankfully, she told me about it and we got it sorted through school. But she still worries that once it’s gone online it’s there forever.”

By its very nature, the internet has always been tricky to police. It was meant to be a free-for-all and is the greatest facilitator of free speech in the modern world. It has given the public the means to act as the media; posting photographs, reporting on events and revealing scandals that the Press is banned from airing.

Now the laws governing the media are having to play catch-up – and policing trolls is also on the agenda. The issue has been discussed in Parliament this summer, with possible amendments to the Defamation Act being considered. To do this properly, there is a need to be clear about what a troll is.

Are you only a troll if your comments include racial abuse, death threats or distress to the grieving? Or does calling someone ugly make you one?

Psychology professor Craig Jackson thinks the difference is vast.

“Deliberate trolls, who target grief sites for example, are different from your casual web user who may post negative comments,” he says. “With Twitter, people see a chance to comment and, when they see the response to it, they’re filled with instant gratification.

“For some people, the web is an extension of real life and they will type things they’d say anyway. The trouble is, people forget you’ve got a much wider audience on the web and might not realise the impact of what they’re saying. Society is only just beginning to catch up with the potential damage of all this.

“If you phone somebody up and threaten their safety, that’s a criminal offence, and the same applies if you say those things online.”

He adds: “The good thing about Twitter is that, though it makes it very easy for people to post unfortunate comments, the Twitter community has become very good at self-policing.

“Users are very active in reporting people who post abusive or offensive comments, getting them banned or bringing them to national attention.”