One who posts a deliberately provocative message to a newsgroup or message board with the intention of causing maximum disruption and argument.
A person who, on a message forum of some type, attacks and flames other members of the forum for any of a number of reasons such as rank, previous disagreements, sex, status, ect.
A troll usually flames threads without staying on topic, unlike a "Flamer" who flames a thread because he/she disagrees with the content of the thread.
And I thought they were drafts who hid under bridges and jumped in front of oncoming travelers to charge then for their passage. And that is how these roads came to be called troll roads. Now I find out I have been wrong all these years.