Trolls are people with small brains and large amounts of free time.
They like to come to forums and post silly stuff to try to get people to get all concerned about them, or to get defensive, or just to get them wound up. They are usually kids, or at least mentally they are.
Trolls give each other props (called LULZ) for being the first to post on a new forum, or to get the longest thread going, or just for winding up the most people about something.
Biggest problem, aside from the general nuisance factor, is that all too often, troll posts get picked up by bots and spread across the internet as "truth" -- so, for example, a fake posting about eyestrain that generates a lot of responses gets picked up and put into some other forum or site, and next thing you know there's a headline on CNET saying "Kindle users report serious eyestrain problems"
Oh, and trolls get big props for having that happen.