Around the parts of the internet from where I 'come', the term 'troll' is used to describe someone who finds amusement in argueing either for or against anything in order to stir up confusion and stark reactions. Thus, a 'troll' will pick whichever side with the most controverial viewpoint, and argue blindly until people grow tired of him/her and stop responding.
However, I find it highly immature to go about yelling "troll!" to someone as soon as he/she is advocating controversial viewpoints, or seeming to be unreasonable in an argument. Everyone does not have the same definition of 'reason', and no one of us that walk the earth today can be entirely objective. Personally, I think that the real 'trolls' that are out there are quite rare, and that most people who engage in discussions on the internet do it because they are genuinely interested - so I cannot quite see how anyone can loose but the 'troll', investing time and energy only to stir up a fuzz, and argue for anything however wicked just to do so.