Trolling is saying something off topic e.g:
Title: How do i earn merits?
User1: i think that you need to write something useful in the comment section.
Troller: Oh By the way guys, have you seen that movie that is out? i think it is called red 1234 or something like that.
No. That is being a moron and offtopic, and kinda spammy.
Not trolling.
Trolling, in all its simplicity, would be to write something to spark anger, sadness or other negative emotions, and doing so hoping for those reactions to occur.
Rarely done properly, as most miss the main goal of getting others banned without getting banned yourself.
Some might consider trolling to be, say, post in a lot of really old threads and wonder if that will piss someone off. It's not trolling, it is just stupidity.
Making a hundred accounts breaking the rules and posting obscene content on the site, just to get the accounts banned over and over is not trolling either, it is account suicide.