A person who visits a site and causes mayhem and disorder within the site in order to either get attention or prove a point through disorderly conduct or rudeness. Spamming, flaming, and hacking are some of the more common tools that a troll uses.

I'm sure that the person never meant any harm to you; Animation Source was still in the process of "detrolling" by a few major trolls when you came and you happened to have gotten caught in the crossfire (you weren't a member at that moment so upon first suspicion, you might have been perceived as a troll. When you did become a member and did not exhibit any signs of trolling, suspicion wore off and you were accepted into the AS family.)

It just means a rude person.