http://cgi.www.differentstrokes.co.uk/cgi-bin/www.differentstrokes.co.uk/config.pl?read=3391

Allow me to quote the pertinent portion of the link:

"Trolls live under bridges and come out at night to frighten and attack people, and possibly to eat them. I believe they originate in Norse folklore (maybe Gerrie can enlighten us here), and am not certain how this term came to be adopted on the Internet. On message boards and news groups a troll is an inadequate person who deliberately posts a provocative message, waits for someome else to respond, and then launches personal attacks with abandon. For some strange reason this gives them pleasure when what they really need is psychiatric assistance.""

Mystery solved.