I know what a "troll" is but on fanpop, I've seen users refering to some others as those. What kind of stuff does someone have to do to be called a troll?

A troll is someone who disrupts the peace of an online environment. Like, someone who spams things with useless প্রশ্ন and comments. Normally doing something like that once won't earn আপনি the name 'troll' but if someone repeatedly posts things seemingly with no reason other than to annoy people, they're called trolls.
Over-zealous haters could also be referred to as trolls if they get too annoying. For example, it's fine to hate Britney Spears, but if a person goes on the Britney Spears ফ্যানপপ page মতামত on যেভাবে খুশী things saying stuff like, "BRITTNY SPERZ SUKZZZZZ!!!!" They would be trolling.
Someone could be counted as a troll for posting strange, irrelevant প্রশ্ন on a specific ফ্যানপপ page. That isn't too bad, especially if it's the যেভাবে খুশী spot, but if they repeatedly post non-questions in the উত্তর area, and it starts to annoy others, that would also be trolling.
Does this help?

A troll is someone on a website, such as Fanpop, YouTube, ect, whose whole reason for getting on there is to piss everyone off for their own sick enjoyment. They may not even belive what they're saying, usually they don't.

They often use incorrect grammar.