A troll comments in a blog or internet forum so as to cause trouble for the community and general readership.
A regular troll typically posts inflammatory comments in a blog in order to provoke a reaction from the regular readers. Often the troll will respond endlessly to any responses with further remarks designed to keep the furore going - until they are either banned from the site or others stop responding. Hence the phrase "Don't feed the trolls".
Other species of troll that are sometimes encountered on Pharyngula include:
A concern troll pretends to be a general supporter of the site, but they have "concerns". The idea is to undermine the consensus viewpoint by pointing out that other commenters or the site may be getting themselves in trouble in some way. They identify problems that don't really exist and offer "helpful advice" - which, if acted upon, would actually work against the purpose of the site and general readership.
Concern trolls are fairly easily identified, and because they are trying to disrupt surreptitiously will frequently depart the site when directly challenged.
See also tone troll
Tone Trolls are the language puritans of the blog world. They will studiously avoid addressing the substantive issues of an argument, but will tut-tut at the tone of the conversation or the language used. They are also easily "offended" by not treating their pet opinion with the automatic respect that it apparently deserves.
Tone trolls can be sent on their way with a well-placed "fuck off", but often not without a final departing snipe at how rude and aggressive people are around here.
See also antifeminist trolls
Any topic that touches upon feminism on Pharyngula is guaranteed to start a flame war the likes of which even God has never seen. The patterns and arguments are repetitive enough that they can be classified into common types.