Other than being Internet harassers, trolls have their origin as a name for several different (depending on region, time) humanoid creatures, inspired by and originating in Norse mythology. Troll may have originated as a negative synonym for jötunn, a mythological race that were often in opposition or competition with the Norse gods (the Æsir).

Their physical appearance is varied; while in Norway they might be more associated with giants, specifically as depicted in the art of Theodor Kittelsen[1]; in Sweden, the work of John Bauer[2] seems to have influenced the general perception of trolls in a similar way; in Middle Sweden, trolls apparently look very much like humans, to the point that it is hard to distinguish them from humans; outside of Scandinavia, troll dolls[3] may the most known depiction.

To troll (Norwegian: å trolle) was apparently related to the verb å trylle: to do/practice magic. Trolldom is a word which in contemporary Norwegian simply means magic (with IMO a more supernatural connotation than the word magi), while it was a legal term to denote witchcraft at the time of the witch-hunts. Maybe to troll can be related to the more modern meaning of being a deceitful trickster in anonymous forums? There is also a saying, å gå troll i ord, which can convey the same sentiment as "be careful what you wish for". More generally, it is a superstition of not daring to speak of some evil by its rightful name, for fear evoking/summoning it (like... Voldemort in Harry Potter). A Swedish dictionary from 1678 has this to say about that matter: «när man talar om trollet, så är det inte långt borta» ('the troll is not far away when one speaks of it').

Trolls seem have an aversion to human settlements, preferring to live in remote and "uninhabited" places, like forests, mountains and caves. In Southern Sweden folklore, trolls seem to be believed to have either died out or driven away with the rise of Christianity[4]. One still had reason to fear the trolls, though, since they could kidnap you and drive you mad (bergtagning), which was used as an explanation of mental illness. They could also swap your child for their own (changeling). Thankfully, trolls could be driven away by the sound of church bells, in the same way that they had an aversion to Thor and his hammer. A somewhat famous phrase from Norwegian folklore is a troll exclaiming that "he can smell Christian man's blood!", which I think was supposed to mean that he was fond of eating Christian men (humans?).

Trolls were also known for hurling rocks, for example at rocks. I think this has to do with seemingly random large rocks that can be found in the North Northern Hemisphere where they "couldn't have ended up naturally", while in reality they were carried there by glaciers thousands of years ago. Some trolls are also part of the Scandinavian nature itself, due to their particular weakness of turning to stone at the sight of sunlight.

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A work totally unrelated to "The Troll Hunters" is "The Troll Hunter", a Norwegian mockumentary about a man who hunts actual, mythological trolls.

[1] See (google) the painting Trollet som grunner på hvor gammelt det er

[2] http://no.wikipedia.org/wiki/Troll#mediaviewer/File:John_Bau...

[3] http://en.wikipedia.org/wiki/Troll_doll

[4] This is something they have in common with some "elvish" creatures in Norse mythology; they were also seen as evil when Christianity took a foothold. But unlike trolls, these elvish beings were previously thought of as good beings.