Do you know how fishermen hang a line in the water when their boat is going really slow? That is called trolling. They are throwing out bait hoping for a bite.

Trolls on the internet are people who post things meant to incite emotional reactions. They don't necessarily have a specific target at first, but they will learn what upsets any given community. Any reaction to a troll scores them points. Usually if everyone ignores them they will go away and find people who get upset easier.