It's a meaningless buzzword like proactive and terrorist. Don't ask me why. It just is.
Hardly meaningless. A motivated troll can wreak a lot of havoc if they aren't nipped in the bud. They can RUIN the sense of community in a forum, they can split cohesive groups into warring factions, in short, they can do a lot of damage.
A troll is deliberately provocative. There are lots of different types of trolling behavior. Making a flame-bait post is just one method.
Some like to be sympathy sponges, posting all their "woe-is-me" stories in a bid to get attention, or maybe more.
Some like to sock puppet (creating multiple accounts/profiles). Sometimes they use the sock puppet to support their own unpopular posts, sometimes they use a sock puppet to COUNTER their posts and create drama where none existed.
True trolls are in it for their own amusement. They just want to throw out something that people will get fired up over, then chuckle when someone takes the bait.
I watched the forum at Snopes almost implode because of one troll a few years ago. Google "Snopes Kyoko", it's documented all over the place. One person had a pretty big effect on that forum. I swore off posting on forums after that, and it was only after much hesitation that I signed up here. It's an experience that has stuck with me simply because of its magnitude.
Thankfully, I haven't seen anything like that here (although some areas are snarkier than others). Some grumpy people, some confrontational people, but... I try to avoid the flamey stuff when I see it. I think the mods do a pretty good job of keeping it down to a dull roar.
If you see something that seems like it might be a little troll-y, your best bet is to take it to a mod, and refrain from posting accusations which just add fuel to the fire. Let a mod handle something if you think it's out of hand.