Human behaviour on the internet is significantly different to real life. The ability to build personas, the near total freedom and the anonymity available changes people when they log on.
As a result of this we are left with a zoo of strange creatures which are rare finds in reality, bible pushers, nazis, camwhores, the list goes on and on. The bottom line is that the freedom of expression breeds ignorance and intransigence, on the internet I have no shackles, I do not have to listen to you or respect you, my opinion is ultimate.
At least, that's how a few people act. These people commune in various sections of the great internet zoo and smear each other with their own ignorance. Enter the troll.
Your average troll is a white male between the ages of 16 and 19 with the more experienced trolls trolling well into their 20s. There is a plethora of reasons one wishes to troll but they all get the same kick of out of it, well some have other motives i.e. Anon but the kick remains.
The methodology is simple, exploit the stupidity and ignorance that plagues this zoo through any means available.
Average trolls looking for easy targets will hang around gaming forums looking for fanboys, Yahoo! Answers, vulnerable individuals such as Mumsnet and MLP forumgoers.
Then you have more skilled trolls who have better things to do than post shock images and outline why the PS3 sucks, their MO and targets will vary greatly, but they understand that as an individual they're weak, a minor inconvenience for a forum moderator. And so they'll band together, and usually go for targets more ambitious than forums, and can spend weeks planning every aspect of their mission.
However, for all the thought and planning, all trolls can be defeated by knowledge, for the lack of knowledge is exactly what they need to troll. In the end the old proverb stands:
"Please do not feed the troll"
...it really is a zoo out there.