The delicate art of fucking with people's emotions or perceived conception of reality, logic or social norms either over the internet or IRL by adopting butthurtful behaviors and/or speech with the sole purpose of lulz. What is true to say though is that trolls are insecure people who want to boost their hurt confidence (often done by other trolls) by asserting their dominance over people they consider sensitive or at least exploitable enough to being hurt. Otherwise, trolling can be viewed as exercising overkill in verbal or visual sadism.
ENCYCLOPEDIA DRAMATICA. That is what you only need to know if you want to know what trolling really is all about.
Our Encyclopedia Dramatica in heaven,
hallowed be your name.
Your kingdom come,
your troll be done,
on internets as it is in heaven.
Give us this day our daily lulz,
and forgive us our 4chan boards,
as we also have forgiven our haters.
And lead us not into exposure,
but deliver us from fail.