A troll, at least in GreekChat parlance, is someone who basically spams the boards, posting the same topics over and over and over....again. Trolls also ask inane questions of the GC general population that would be better answered by members of their own organization. Trolls also tend to lie or, at least, make questionable statements. Then when they're questioned, they make up excuses (nonono, I meant THIS, or you misunderstood me). If you read a question from someone and the first thing that goes through your head is, "Is it really possible that this person doesn't know this????", chances are you're reading a post from a troll.
I think a lot of people on GreekChat are cautious because there have been posters in the past who aren't who they claim to be. Not that that's any big deal in the general scheme of things, but no one likes to be lied to.
Hope this helps clear things up!
PS moe.ron, who told you you could post that picture of my mother-in-law???