A troll is someone who posts simply to anger and with no intention of contributing. It can be as blatant as someone posting "Racial slur Racial slur Racial slur Racial slur Racial slur Racial slur Racial slur Racial slur Racial slur!", or as subtle as someone posting "Popular thing is dumb and the people who like it are dumb". Someone who posts an opinion you disagree with, is not a troll however.
I agree with this, however I consider anyone who disagrees with you in a needlessly vitriolic and/or condescending as well as unnecessarily agitated manner to be a troll as well.
Person 1: "I disagree with you. Here's why..."
Person 2: "Sigh. Here's why you're wrong... . Now go away and let the adults talk."
Person 3: "You fucking ******! You're fucking wrong you piece of shit, go die in a fucking fire or something because you know absolutely nothing about what you're talking about, you fucking moron."
In my view, Persons 2 and 3 count as trolls.