Being a ******* on the internet is not trolling, it's just being a ******* on the internet.
Trolling is posting stupid/controversial/odd things in order to get a reaction from people and seeing how many people you can string along before everyone realises what you're doing.
You see it most often on here in the guise of people holding patently absurd positions in arguments just to attract the righteous fury of everyone else.
A good recent example was the guy who got the Macbook/iPad etc. for Christmas and posted saying 'Unhappy tbh, was expecting a car' and had a load of people firing off 'OMG you spoilt git'. That was a good troll, if a little obvious. It wasn't even remotely true, he was perfectly happy with what he got but he still hooked a fair few people into reacting because of it
Just abusing and insulting people though, that's not trolling, that's just being a knob.