Wikipedia defines trolling here. http://en.wikipedia.org/wiki/Troll_(Internet). and i agree with it.
'Abuse' is some one, any one, me for example disagreeing with your views but instead of sharing my logic or my point i would pick your race, gender, religion,nationality , physical attribute or any point of your personality or profession/trade and just make degretory remarks about it. Remarks that would not have any relationship whatsoever with topic or discussion but i would say them cause i have nothing to contribute. Every body is entitled to their opinion. We should respect that. This does not stop controversial but intelligent discourse as stated above. Again some people will not agree with it. So they are entitled to.