They're the pitchfork mob or football streaker of the digital age - internet ''trolls'' who derive pleasure out of targeting people with abuse or pranks and watching them react.
The TV personality Charlotte Dawson felt the full force of online hate this week after she tried to name and shame an online abuser who had told her to go hang herself.
But this is just the tip of the iceberg, said Dr Emma Jane, a senior lecturer at the University of NSW who analysed online aggression for her PhD and now studies it as her central area of research.
''These things have always existed but the internet is the perfect environment to let them manifest in their ugliest extreme,'' Jane said. ''I've watched it begin as the odd revolting comment in the odd corner of the cybersphere in the late '90s to now, where it's everywhere all the time.''
For some, trolling gives a sense of power over the target, a sense that may be lacking in their real-world lives. For other trolls who gather on websites such as 4chan.org and Encyclopedia Dramatica, attacking or ''flaming'' people is a game.
The big mistake many - including Dawson - make is responding to the attackers and giving their remarks oxygen (a popular internet term is ''don't feed the trolls'').
''A lot of the rationale is that anyone that takes abuse online seriously deserves to suffer because it's just words and it's just playing,'' Jane said.
Mobs will often compete to see who can say the most revolting or politically incorrect thing and popular female personalities are frequent targets of misogynist abuse.
The writer and TV identity Marieke Hardy this year tried to name and shame who she thought was the author of a hate blog about her and ended up naming the wrong person, who is now suing Hardy and Twitter.
But it is not just the internet fringe who are trolling - if you've fired a personal attack at someone while making an argument in the comments of an online news article or YouTube video, chances are you have ''trolled'' too.
''The moment you disagree with someone online it's become common practice to engage in this really over-the-top ad hominem invective where you play the man, or woman, rather than the argument,'' Jane said.
Venessa Paech has been a professional web community manager since 1989 and is the co-founder of the Swarm community management conference. She said trolls were people who ''crave attention and crave social theatre''.
''Most community managers I know have had death threats directed at them personally,'' she said.
''We get to see the best and the worst of people.''
Paech said that trolls were often middle-aged men who were ''angry at the world'', but one she encountered in her research was particularly unusual. Someone had been tearing down virtual spaces and bullying other users in the children's online world Habbo Hotel.
''It ended up being a 12-year-old girl in a wheelchair who was horribly abused in her offline life and this was a way of making herself feel better and heal,'' Paech said.