My understanding of a troll is a fake poster attempting to trick people into believing they are a true poster and assuming to receive a lot of responses so they know they've tricked a lot of people. They will usually try to post things they know will get a lot of attention and responses, often with the deliberate intention to cause those who read it to feel angry/ upset/ offended and to try and cause an extreme reaction to their thread/post.
This might mean someone posting abusive comments or starting threads about sensitive topics or airing extreme views (e.g.: racism, xenophobia). It might mean starting a fake thread, sometimes about something ludicrous and unbelievable but sometimes about things that seem believable at first and draw in a lot of replies. The worst trolls make up stories about very sensitive subjects (e.g.: bereavement, illness) and have huge numbers of real posters share their true stories or offer support before it transpires they're a troll. On Mumsnet common subjects for trolls might be anti formula-feeding, anti-abortion, women-bashing, anti-sahm etc: obviously lots of real posters might also hold these views but a troll will post something deliberately inflammatory around such a subject knowing that it will draw in people on both sides of the argument and likely cause an argument.
The troll basically thrives on replies and attention to their posts and wants to cause as much attention to their post/ thread as possible before they're discovered. Trolling happens on pretty much any website where people can join and post and trolls can range from the annoying but fairly harmless (e.g.: posting about baby names) to the slightly creepy (e.g. poo trolls who will try and get people to share stories about pooing to feed their own poo fetish) to the malicious (e.g. posting racist views, threatening people) to the dangerous (e.g.: posting about losing a child and having posters offer emotional and even financial support believing it's true, targeting vulnerable groups such as rape victims or people with mental-illnesses and trying to make them feel blame/ shame etc).
As with anything people who start off trolling in a fairly harmless way can easily get a taste for it and start to post threads which will harvest more and more responses (either by causing people to feel more anger, more upset or more sympathy) hence the need to take a zero tolerance approach to all trolling.