I understand it to mean 'deliberately trying to provoke'. Usually in an off topic sense.
So, if a thread is discussing bananas, and you are passionate about how bananas ought to be, you can expound at length on bananas. Even if you know people will take issue with you about it (as long as you are somewhat polite).
But if the thread is about 'favourite colours' and you use yellow=bananas and start ranting about bananas, basically looking for someone to argue with you, cos you know another poster feels strongly about them too - that's trolling.
More common examples are turning every Civ3 thread into a "why Firaxis s***s" thread, or every modern politics thread into a 'the mideast is the "(insert race/tribe here)'s fault" thread