A troll is a mythical creature, like a giant or a dwarf.
A troll, in Internet parlance, is an entity or character that purposely disrupts a conversation via usenet or email, a comment thread on a blog, or another sort of website. For example, on Wikis, users who purposely make disruptive edits are considered trolls. The term originated in the early 1990's, an abbreviation of the term "trolling for suckers" (trolling being an angling technique to seek fish).
An Internet troll will lock their target, bait the line and spread the net to catch their goal (unwitting victim).