Please don't feed the animals!
In Internet terminology, a troll is a person who posts inflammatory messages on the internet, such as on online discussion forums, to disrupt discussion or to upset its participants. It can also be used as a verb, meaning to post such messages, and "trolling" (the gerund) is also commonly used to describe the activity.
Some more related links:
Cunningham & Cunningham, Inc.
The Jargon File
For whatever reason, there are a number of individuals who seem to take great delight in disrupting various groups of posters, most often (I believe) due to some character flaw. They may simply be anarchistic, narcissistic or whatever, but their motives are generally suspect. A lot of them seem to feel as though they are intellectually superior to most others, whether this is valid or not. Sometimes, people are accused of being trolls when it is really not their intention to be such, but time will generally tell.
In any case, the best advice is "Do not feed the trolls"...