According to Wikipedia, a troll is, "someone who posts inflammatory, extraneous, or off-topic messages in an online community, such as an online discussion forum, chat room, or blog, with the primary intent of provoking other users into a desired emotional response or of otherwise disrupting normal on-topic discussion."
In other words, a troll is anyone whose only intent in posting is to make you mad.
 Advice on how to deal with trolls
Don't feed them. It's as simple as that. Ignore a troll and they'll wither and die.