1.An Internet troll, or simply troll in Internet slang, is someone who posts controversial and usually irrelevant or off-topic messages in an online community, such as an online discussion forum or chat room, with the intention of baiting other users into an emotional response or to generally disrupt normal on-topic discussion
In academic literature, the practice was first documented by Judith Donath (1999), who used several anecdotal examples from various Usenet newsgroups in her discussion. Donath's paper outlines the ambiguity of identity in a disembodied "virtual community":
In the physical world there is an inherent unity to the self, for the body provides a compelling and convenient definition of identity. The norm is: one body, one identity. ... The virtual world is different. It is composed of information rather than matter.
Donath provides a concise overview of identity deception games which trade on the confusion between physical and epistemic community:
Trolling is a game about identity deception, albeit one that is played without the consent of most of the players. The troll attempts to pass as a legitimate participant, sharing the group's common interests and concerns; the newsgroups members, if they are cognizant of trolls and other identity deceptions, attempt to both distinguish real from trolling postings, and upon judging a poster a troll, make the offending poster leave the group. Their success at the former depends on how well they — and the troll — understand identity cues; their success at the latter depends on whether the troll's enjoyment is sufficiently diminished or outweighed by the costs imposed by the group.
Trolls can be costly in several ways. A troll can disrupt the discussion on a newsgroup, disseminate bad advice, and damage the feeling of trust in the newsgroup community. Furthermore, in a group that has become sensitized to trolling — where the rate of deception is high — many honestly naïve questions may be quickly rejected as trollings. This can be quite off-putting to the new user who upon venturing a first posting is immediately bombarded with angry accusations. Even if the accusation is unfounded, being branded a troll is quite damaging to one's online reputation." (Donath, 1999, p. 45)
The term troll is highly subjective. Some readers may characterize a post as trolling, while others may regard the same post as a legitimate contribution to the discussion, even if controversial. The term is often used to discredit an opposing position, or its proponent, by argument fallacy ad hominem.
Often, calling someone a troll makes assumptions about a writer's motives. Regardless of the circumstances, controversial posts may attract a particularly strong response from those unfamiliar with the robust dialogue found in some online, rather than physical, communities. Experienced participants in online forums know that the most effective way to discourage a troll is usually to ignore him or her, because responding tends to encourage trolls to continue disruptive posts — hence the often-seen warning: "Please do not feed the trolls".
Frequently, someone who has been labelled a troll by a group may seek to redeem their reputation by discrediting their opponents, for example by claiming that other members of the group are closed-minded, conspirators, or trolls themselves.
3. Concern troll
A concern troll is a false flag pseudonym created by a user whose point of view is opposed to the one that the user's sockpuppet claims to hold. The concern troll posts in web forums devoted to its declared point of view and attempts to sway the group's actions or opinions while claiming to share their goals, but with professed "concerns". The goal is to sow fear, uncertainty and doubt within the group.
For example, in 2006 a top staffer for then-Congressman Charlie Bass (R-NH) was caught posing as a "concerned" supporter of Bass's opponent, Democrat Paul Hodes, on several liberal New Hampshire blogs, using the pseudonyms "IndieNH" or "IndyNH." "IndyNH" expressed concern that Democrats might just be wasting their time or money on Hodes, because Bass was unbeatable.
A recently declassified World War II manual on sabotage recommends such techniques to derail any effective action: "Advocate 'caution.' Be 'reasonable' and urge your fellow-conferees to be 'reasonable' and avoid haste which might result in embarrassments or difficulties later on... Be worried about the propriety of any decision — raise the question of whether such action as is contemplated lies within the jurisdiction of the group or whether it might conflict with the policy of some higher echelon." OSS Simple Sabotage Manual, pdf
Although the term "concern troll" originated in discussions of online behavior, it now sees increasing use to describe similar behaviors that take place offline.
For example, James Wolcott in Vanity Fair  accused a conservative Daily News columnist of "concern troll" behavior in his efforts to downplay the Mark Foley scandal. Wolcott links what he calls concern trolls to Saul Alinsky's "Do-Nothings," giving a long quote from Alinsky on the Do-Nothing's method and effects:
These Do-Nothings profess a commitment to social change for ideals of justice, equality, and opportunity, and then abstain from and discourage all effective action for change. They are known by their brand, 'I agree with your ends but not your means.'
In a more recent example, The Hill published an op-ed piece titled "Dems: Ignore 'Concern Trolls'." Again, the concern trolls in question were not Internet participants; they were Republicans offering public advice and warnings to the Democrats. The author defines "concern trolling" as "offering a poisoned apple in the form of advice to political opponents that, if taken, would harm the recipient."