When they call someone a troll on the Internet, at least in the sense that I use it, it generally means they come off as extremely annoying and they are generally was than a douchebag. They do this at the expense of other peoples' patience because they think it's ridiculously hilarious.
For example, one time there was a troll on here that thought it was just fine and dandy to make egregious remarks about my little sister. Now I don't remember what this particular troll said but then again, why would I want to remember it? :)
A troll is someone who posts an intensionally provocative message intended to start a flame war between participants. Last time I was accused of being a troll I posted an article on a photography forum about the Jock Sturges case and a huge flame war ensued about free speech, child pronography, radical feminism, art vs. pornography etc. I did not consider it off-topic but it sure did kick a hornet's nest.
Miguel answered the question but I would like to add something which people may like to know.
Trolls are usually very easy to get hold of if you search them on the internet and I remember a friend who got one on his blog and searched them on google and found their number and asked why he said all those horrible and mean stuff and the troll just said sorry :L
Miguel pretty much explains it, it's attention seeking for one's own pleausure, people will post things, however outlandish to get a reaction out of others, then site back and watch people replying to them, usually they fuel it even further by coming back.
Trolls in other words are cyber bullies but Miguel pretty much expalined it
A sneaky person who loves to start mo' drama than yo' mama on the internet.